Ladies and gentlemen,
I would like first to explain how my interest was led from my old field of cosmic ray to this new one of rock magnetism.
The steps were few and direct.
The two outstanding problems of interest in the cosmic ray field are the creation of the strange particles
which Professor Heisenberg was discussing a few days ago.
The other exciting problem is that of the origin of these remarkable rays that reach the earth from outer space.
Now, we know very little about the origin of the cosmic rays, except one thing is fairly certain:
They are accelerated somewhere in the universe by rapidly changing magnetic fields.
In fact, by electromagnetic induction.
Consequently, the seeking for an origin of cosmic ray means a seeking for the magnetism in the universe.
The magnetism of stars, the magnetism of moving dust clouds and the magnetism of ex-supernovae like the Crab Nebula.
Now, the origin of these magnetic fields, that are playing so big a part in cosmology generally,
is still somewhat of a mystery.
We know that some stars are extremely highly magnetic and change their magnetism very rapidly.
And there is today no acceptable theory of how this happens.
Turning to the earth, we know the earth is a magnet,
and this fact has had an enormous effect on the history of civilisation.
It is just because the earth is a magnet that the early navigators were enabled to make their long voyages
and so to bring the new world into the kin of the old.
But we know, in fact, not much more about why the earth is a magnet than did William Gilbert
when he published his great book on the magnet in 1600.
One of the difficulties formulating a theory of the earth's magnetism
is that we only have historic records of the compass needle and the dip needle for about 400 years
and in very few parts of the world.
Now, it's long been recognised that the magnetism of rocks provides a possibility of tracing back the magnetism of the earth,
back to geological, right through geological past.
At least to 500 million years old and probably, eventually, to 2,000 or more million years old.
This comes about, because rocks are, in fact, nearly all slightly magnetic.
The different sorts of rocks get their magnetism in different kinds of ways,
and these processes are extremely complex and I will have to go into these in a little detail in a few minutes.
But for the moment I want to emphasise only that many rocks, not all unfortunately,
when they are formed, either by cooling volcanic magma
or when they're formed by deposition of sand and silt in the river or sea,
acquire a magnetisation in the direction of the ambient field.
This intensity of magnetisation is roughly proportional to the ambient field and in the same direction.
Thus, by studying the rocks, cutting pieces out of the rocks,
taking them to the laboratory and measuring their weak magnetism, both in magnitude and direction,
we can deduce something about the history of the earth's magnetic field back to the beginning of geological time.
The subject, the study of rock magnetism, is not a new one.
It goes back at least 50 or more years.
It was pioneered by Koenigsberger in Germany, by the French, Italians and in Japan.
But it's only recent years that decisively new and exciting results have been obtained.
The breakthrough, to use that modern Americanism,
came with some work from the Department of Terrestrial Magnetism in Washington in 1948.
When they found that by the study of the magnetism of the varve plays,
the glacial plays of New England, that were laid down at the session of the last ice age in Pleistocene time,
they could trace back the intensity, the declination and the dip of the earth's magnetism,
actually their particular work from 10,000 BC to 14,000 BC, year by year.
Because, as you all know, these plays are like the rings of a tree laid down each year with the seasonal variation.
This beautiful work first put rock magnetism on the map as a way of producing real information about the past.
This was the work that led my group in Manchester and then in London
to get interested in tracing back the history of the earth's magnetic field into time,
so that we could have more data, experimental data, to approach the problem of why the earth is a magnet.
Now I'll just break off for a moment to say something about what is believed to be the origin of the earth's field.
The inside of the earth, the centre, half in radius,
is believed to be a liquid core of conducting material, probably iron alloy.
And it is supposed that convection currents occur in this core due to the radioactivity.
These convection currents are supposed to interact with the rotation of the earth through Coriolis forces
to produce extremely complicated three-dimensional movements,
which are an extreme case of the subject of magnetohydrodynamics.
And it is hoped, by the optimist, that Larmor's original suggestion,
that the earth's field could be due to a self-exciting fluid dynamo in the liquid core, will in fact eventually be realised.
Actually, not very much success has yet been made in obtaining an acceptable theory.
The subject is extremely complex, we don't know very much about the inside of the earth,
and I don't think an early outcome can be expected, as to the fundamental theorem which is required,
as to whether a three-dimensional self-exiting fluid dynamo is, in fact, a possibility or not.
But one accepts that as the most likely explanation, which has to be pushed as far as possible.
Now, if one had speculated at what one might find,
if one started to trace the earth's magnetic field backwards by the study of rocks,
one would, in fact, have probably guessed
that the earth's field would have been in about the same direction as at the present day,
but might well have been much bigger in the past or perhaps smaller.
Various theories of the decay of the earth's field have been put forward by Lamb and others.
As so often happens in experimental science, quite other things were found.
The first fact was that recent rocks, recent in geological time, say the last 50 million years,
are magnetised roughly along the earth's magnetic field as it is today.
Except for one astonishing discovery, about half the rocks are magnetised in exactly the opposite direction.
I will show you slides in a moment, showing the actual experimental fact.
About half the igneous rocks, I'm mainly talking about lavas now,
of Tertiary time are magnetised along the direction of the field, and half in about the exact opposite direction.
This phenomenon, the reverse magnetisation, is a matter of acute controversy at the present moment.
There is quite strong evidence, or there's plausible grounds for believing,
that these reversely magnetised rocks may, some of them,
have been caused by an actual reversal of the earth's magnetic field.
That is that the earth's dipole, periodically in geological history,
switched right over through 180 degrees and changed its sign without changing its magnitude.
The reason why many people believe this is that, so far,
in most cases it has been impossible to find any physical or chemical difference between the rocks, at any one place,
which are magnetised normally and those that are magnetised reversely.
In other cases, differences have been found and can be attributed to physical-chemical processes.
There's a rather dramatic, almost, history, there's a little known about these reverse magnetisations.
Firstly, they were discovered originally as far back as 1906, but almost entirely ignored by subsequent workers.
About 1948, Graham, in Washington, one of the pioneer workers in this field,
discovered some rocks which were partially magnetised one way and partially the other,
and he argued this could not be due to reversal of the earth's field, it must be due to a physical-chemical mechanism.
So he wrote to Professor Néel, of Grenoble, and asked him if this was possible,
and Néel produced his famous four possible mechanism.
Within 18 months, two of them had been discovered,
one in the field and one artificially by Gorter in the Philips Laboratory at Eindhoven.
One of the most beautiful predictions in the field of solid state physics.
And we now know half a dozen mechanisms by which rocks can become reversely magnetised.
And yet, there are a large number of phenomena, observations, which are extremely hard to explain by such mechanisms.
So, for the moment I will take up a strictly academic attitude of caution and say that,
as the evidence for the earth's field reversal is strong,
I personally do not think it is conclusive and must await further observations.
But most of my colleagues in the subject believe that it is nearly proved.
A second discovery was that if you take older rocks, Mesozoic rocks, 100 million years or so old,
they are not magnetised, in general, along the direction of the present field.
And this indicates that that particular piece of land must have moved, relative to the pole, since it was formed.
The assumption here made is that on the average, over a long geological time, say 10 million years,
that the magnetic axis of the earth is coincident with the rotational or geographical axis.
There are very strong theoretical reasons why this should be so, on the average,
and there is quite a lot of experimental evidence that it must, in fact, have been so,
but it will be put to the test by the type of experiments which I am going to describe.
Now, just a moment to explain what we can deduce from the magnetism of a rock.
You all know the round-like shape of aligned magnetic force around the earth.
At the equator they're horizontal, at the pole they're vertical, in between they're ... (inaudible 12:37).
If the rock is magnetised at the equator, when you pick it up,
supposing it still retains its original magnetisation, it will magnetise horizontally.
At the pole it will magnetise vertically downwards, South Pole, the other way around, upwards.
Intermediate latitude, at a given angle, forms a dip.
It's perfectly easy to calculate that the magnetic latitude is half the tangent of the magnetic dip,
which allows us, from the measurement of the dip of a rock, to deduce the magnetic latitude, when it was formed,
on the assumption that no disturbing influences had, in fact, happened.
We cannot, of course, tell the longitude, for the very good reason that, while deities created the poles,
but man created rings, so you cannot tell where the rock is, east or west,
for the simple reason that is a dimensional matter.
So we cannot tell east-west movements of continents, but we can tell north-south movements.
We can also tell angular changes.
If we find, the magnetisation is not along the northern meridian, but at an angle to it,
we can argue that the land must have rotated that way or that way since it was, in fact, formed.
That is a type of information that will be obtained from measurement of the azimuth
and the dip of the magnetisation of rocks.
And this information is practically independent of whether the earth's field has reversed,
or by, really, in fact, or whether reverse rocks are reversely magnetised by physical-chemical means.
We just occasionally get an ambiguity, that's very seldom indeed.
Now, the first slide will show you typical results of the study of some igneous rocks,
United States by Runcorn's, Cambridge, these are the Columbia River Basalts.
Now, we are used to showing our results in the following way.
We want to have a projection which will show the direction of magnetisation of a rock specimen,
so we imagine the hemisphere, horizontal hemisphere, and we look downwards,
and this point here represents the direction downwards, this point represents horizontal north, south, east and west.
This is a variant of the Schmidt stereographic projection.
Now, the present direction of the earth's field in the Columbia River, America, is there,
a little east of north, the destination is east, and dipping downwards at an angle of some 60 degrees.
Now, these black dots represent rock specimens from the lava flow of the Columbia River,
and you'll see, the top of them are grouped roughly around the present direction of the field.
But part of them are in the exact opposite direction.
These open circles are projected on to the top hemisphere,
so that that direction and that are practically 180 degrees apart.
This is typical of the finding that Tertiary rocks,
that is, we'll say rocks from 50 million years old until the present day,
about half are magnetised one way, roughly along the present field,
and a half are magnetised almost exactly the opposite direction.
And the authors of this work conclude from this and other similar evidence
that this provides very strong argument for the reversal of the earth's field, quite frequently,
every million years or so, in Tertiary time.
Now, our work, my group, Manchester under Clegg, started to work, a few years ago,
on the red sandstones of England, the Triassic red sandstones of England,
which outcrop all over Lancashire, and of which so much of our architecture, like Chester Cathedral, is made.
These rocks proved extremely fertile ground for discovery.
This group investigated the new red sandstones from ten sites in England.
From Cumberland to Devonshire, over 300 miles and discovered two very remarkable facts:
They discovered, the rocks were not magnetised north and south, but northeast and southwest,
and half of them were one way and half the other.
I plotted here the destination on a circular histogram.
This is the circular baseline,
these points represent the number of rocks specimen in that direction and this in this direction.
You can see, the mean direction is northeast of half the rocks, and the other half are southwest.
This angle of 34 degrees between the true north and this direction
is the first established case of a really big discrepancy between the magnetisation of a rock and the present field,
and can be taken with some certainty as indicating that England must, in fact,
have rotated 34 degrees clockwise since Triassic times, which was about 150 million years ago.
The next slide will show the results of the dips.
All the rocks which were magnetised northeast are dipping downwards.
That is the radial histogram, showing the mean dip is 34 degrees downwards,
whereas the present field is right down here at 67, this is the mean dipole.
The other half of the rocks, those that are magnetised southwest, are mainly dipping upwards,
but a few are just dipping downwards.
But again, there is the rough 180 degree relationship, we will ignore for the moment this slight discrepancy,
which is due to slight instability of the magnetism.
This diagram shows results which can be interpreted as saying that England was, 150 million years ago,
was at a latitude of about 17 degrees north compared with its 55 degrees at the present time.
England has moved some 35 degrees northward, relative to the magnetic pole, in the last 150 million years.
And for reasons which I've mentioned we will assume, though there is no firm proof of this,
that this also is a movement relative to the geographical pole.
So this diagram was the first to show with great plausibility a major northward land movement, a movement of a land mass.
It's quite easy to work out that if England has moved from 17 north to 55 north in 150 million years,
that works out at about a quarter of a degree of latitude per million years, or about three metres a century.
Something which would be just within the bounds of possible experimental check in the next few hundred years,
if we use every sort of modern precision.
I want now to leave this continental drift question for a moment, to speak a bit about these reversals,
because they take one into an extraordinarily fascinating piece of modern solid-state theory.
You all know, the modern theory of paramagnetism, going back to Weiss and later quantised by Heisenberg,
looks at a paramagnetic body as one in which all the elementary atomic magnets
are lined up together by quantum exchange forces.
Néel pointed out that you can get conditions, which there are two sets of sites, A and B, in the crystal,
at which all the A magnetic moments are antiparallel to the B.
In this case, when they're equal strength,
there is no external magnetic moment and the body is called an anti-paramagnetic.
But in cases where the A and B have different strengths, as they often do,
you get the resultant moment and you get what Néel called a semi-magnetic or an imperfect anti-paramagnetic.
And these have been the subject of great experimental and theoretical study.
Néel showed that a body of this, no, Néel guessed that a body of this sort might exist,
in which the temperature variation of the A and B lattices are what differ.
This way, the magnetisation and the functional temperature, the A lattice fell like this,
the B one antiparallel went like that, and the result went like that.
Which would lead to a magnet, which reversed itself spontaneously at a certain temperature.
It's been 18 months of discovery, Philips synthesised a lithium-chromium ferrite which, in fact, did exactly this.
And now many other are known.
As an example, a picture from Gorter with a magnetisation of a lithium-chromium ferrite series
with different amounts of lithium in it,
and for a certain range of lithium content they reversed the magnetisation at any temperature,
which you can choose according to the composition.
It's a very nice experiment, gets a bit from Dr. Gorter, heat it up in a beaker
and watch it go round at a few degrees above room temperature very often.
This phenomenon has been known in the laboratory, but is not yet known to have a direct application to rock magnetism.
But Néel's third hypothesis has been, this is this one,
whichever can get a laminar structure of into the magnetic component of different magnetic properties,
will have to get reversal phenomena.
When you cool such a body consisting of A and B through the Curie point,
proposing the intensely magnetisation A right about there and B right about there,
the low Curie point and a high intrinsic magnetisation, A magnetises first in the ambient field
and B magnetises in the reverse field due to A.
And when you get down to room temperature, the field is reversed.
This was discovered by Nagata in Tokyo, two years after Néel's prediction
and quite recently he's published, his school,
some very beautiful microphotographs of the actual laminar structure that leads to reversal.
Now you see electron microphotographs of the lamellae inserting rather rare igneous rocks,
which show this reversal effect in the laboratory.
You see the scale is five mu, and these are the lamellae interleaved with another material.
It now turns out that the A and B property, materials, are, in fact,
both solid solutions of ilmenite and hematite of different sorts.
Ilmenite being TiFeO3 and hematite being Fe2O3.
And they make solid solutions with a solubility gap,
and these two lamellae and the intervening medium are both solid solutions of ilmeno-hematite.
The typical results for rocks of this sort, as tested in the laboratory, are as follows:
Here you see rocks heated in the laboratory,
and when they've been cooled, previously large fields 13 earth pits , they behave quite normally.
When they've been cooled as they are in the earth, in weak fields, half a gauss, one gauss,
as you heat them up they reverse in the laboratory.
It's a rare phenomenon, but a quite definite one,
and quite recently in England we found some in the West Country which showed this phenomenon also.
And thus, although this phenomenon is perfectly, definitely established,
it's not very widespread in nature and the great majority of rocks,
which are found reversed in nature do not show this phenomenon in the laboratory.
Thus, we conclude either that the earth's field really has reversed or, and this is a distinct possibility,
I think it's what I really believe in my heart of hearts,
that chemical and physical changes have happened, since the rocks were formed,
to obliterate the former existing reversal mechanism.
I call this an obliteration theory.
One curious incident recently happened, Graham came in to visit me a few months ago and asked me
if I knew that in my laboratory in 1912, a long time before I was there, a Professor Smith had, in fact,
found, investigated and interpreted this reversal phenomenon in steel 40 years before Néel wrote about it.
If you take a piece of steel, annealed steel, magnetise it in the solenoid, take it out the solenoid, you get free poles.
The free poles produce the reverse field in the steel.
The steel, being a mixture of iron and iron carbide, FeC3,
the iron has a low corrosive force and demagnetises under the influence of the self-reversing field, the carbide doesn't.
But the carbide has a very low Curie point, so when you heat it up, the steel nail reverses it.
And here you show Smith and Gill's curve in 1912,
showing what happens when you heat up about 85% carbon steel having been treated in this way,
it reverses its magnetisation at 200 degrees centigrade.
This was identical to Néel's mechanism 40 years before and not known to any of us till Dr. Graham told me a few months ago.
It's a very nice demonstration experiment.
Now I'm going to leave this situation, as I say,
at a truly agnostic academic attitude of "I don't know whether the earth's field has reversed or not.",
and come to this question of the use of rock magnetism for determining earth movements.
Now, the history of continental drift, as it's called, goes back a very long time.
Francis Bacon commented in the early part of the 17th century
on the curious resemblance of the west coast of Africa to the east coast of America
and wondered whether there was any reason for this resemblance.
As long ago as 1858, a French geologist, Snider, in an entirely unknown publication, produced theses maps.
Now you see a map of Africa and South America with the obvious resemblances,
and Snider pushed them together like that and said, they must originally have been like that,
but have drifted apart over millions of years.
This was forgotten until about the early part of this century,
taken up in America by Taylor and in Germany by Alfred Wegener,
who spent a lifetime developing the theory of continental drift.
Now, apart from resemblances of shape, incidentally I should add that Snider showed, as has been very clearly verified,
that there are extraordinary geological resemblances between the African and the American coast,
so much so that some of the mountain ranges can be traced exactly across the intervening gap where you put them together.
But on the whole, I must say straight out that few geologists believe
that these shifts of that magnitude have really occurred.
But quite apart from the possibility of relative shifts of land masses to each other,
which usually goes under the name of continental drift,
quite apart from that, there has been the discussion of polar wandering,
which is the assumption that the crust of the whole, the rigid whole, has moved relative to the geographical axis.
If you imagine, if you take the skin of this globe off and move it round while the axis remains fixed in space,
you would get what we call the hypothesis of polar wandering.
I will come a little bit more at the end to theories of this effect.
But from the point of view of ... (inaudible 30:16),
continental drift is taken to mean the relative motion of continents, polar wandering,
the common motion as a rigid whole, relative to the geographical pole.
Now, polar wandering has been educed very strongly, by many people,
to explain the extraordinary fact of the ancient climates of the globe,
as deduced from geological and paleontological evidence.
For instance, you all know that the coalfields of northern Europe are
in every botanical point of view tropical or semitropical origin and could not be grown in the present climate.
In the same way, the famous laterite belts,
which are definitely of tropical desert origin, are now a long, long way from the equator.
And, as a result of this, the study of palaeoclimatology has suggested changes like this.
Here you see a suggestion for the Carboniferous period, 200 million years ago,
when the coal forests were laid down, Europe, England, Europe comes on the equator.
And this belt, which is now the equator, covers very nice with the coal formation of Europe and similarly in America.
Similarly, you get up here and here, you get the dryer zones with the appropriate type of rock.
Again, this has been an extremely disputed subject
and one that divides geologists into class of those who believe it must have happened
to those who believe it cannot possibly have happened.
In fact, there is a triple antagonism in this subject,
half the rocks are magnetised one way and half the rocks are magnetised the other.
Half the workers in the field believe the earth field has reversed and half believe it hasn't.
Half the geophysicists believe the continents have shifted and half believe they haven't.
It is our job to solve these antagonisms by experiment.
The particular interest to us, in Manchester, as soon as we got this result for the British Trust,
was to go to some place where the palaeoclimatological evidence very strongly suggested a big movement.
And I was extremely struck by this remarkable diagram from Holmes's book Principles of Physical Geology.
Here he traces out the famous Permo-Carboniferous glaciation of the southern hemisphere, that is, 200 million years ago.
It was one of the most remarkable discoveries of geologists, some 50 years ago,
that South Africa and India were extremely heavily glaciated for a very long time.
And by tracing the scratches in the rock and the formation of tillite and other glacial rocks,
it's been possible to draw, in outline, this dotted line,
which represents the northern limit of glaciation of the southern hemisphere 200 million years ago.
South Africa was covered by a large ice cap with the ice flowing under its own weight, north and south.
This is not an ice cap on a mountain, it isn't a glacier, it's an ice cap.
India, astonishing to note, was glaciated with the ice flowing northward, towards what is now the Himalayas,
which were then, of course, not even thought of.
Similarly, South America, Madagascar, South Australia and Tasmania, New Zealand, all glaciated at the same time.
And Holmes in his book, I think, very cautiously, states that this distribution of ice, if correct,
is quite incompatible with the present positions of the continents relative to each other and to the pole.
And unless one could move across the whole crust, round, to bring the South Pole here,
leaving the positions relative to each other unchanged,
but that would mean an enormous ice cap over the huge Indian Ocean and would require an amount of ice
which I don't think there's enough water in the world to produce.
A much more economical method, even if more drastic, is that of Wegener and his pupil Du Toit, of South Africa,
who regrouped fairly arbitrarily these continents around the South Pole.
This is really following on from the original idea, they fit South America and Africa together,
they bring Antarctica here, Africa, Madagascar and Australia, and assume one great ice cap covering the lot.
There are various other reassociations, which are quite arbitrary,
and I don't want you to take any detail at all seriously.
But to an experimental like myself, any theory is a good one if it suggests an experiment.
And the experiment suggested was to get some money from the government to send an expedition to India to see where India was.
(Laughter)
Just before I come to that I want to say that similar measurements to those in England, in the Triassic time,
has been done by Runcorn, Creer and others, right back to Devonian, Cambrian times, 500 million years ago,
and in America by the same group and by Graham in the United States.
And the result would be this.
Can I have the lights on here, please.
In England, the magnetisation is twisted eastwards, and in America it's twisted westwards.
And they both indicate a pole position, 150 million years ago, somewhere near eastern Siberia.
The results for America and England roughly agree, only roughly,
with the concept of a movement of the crust as a rigid whole, relative to the poles.
But there are discrepancies, and the discrepancies are in the sense that it looks as if it is possible
that Europe and America have separated slightly, but not nearly as much as indicated in those old speculations.
That is an answer to the point.
But the American and the British rocks agree in putting the pole, 150 million years ago,
somewhere in the North Pacific, between that and northern, eastern Siberia.
These results have been collected together by Runcorn in the following diagram.
This is a polar view of the globe, and this curve represents various speculation of the movement of the pole,
derived from the palaeoclimatological evidence, which I've already described.
And they all put the pole, a few hundred million years ago, somewhere in the pacific.
The magnetic result is somewhat similar, but rather different result,
and here T is Tertiary, the pole is coming out somewhere here, not far from Japan, between Japan and Siberia.
So that the general movement of the pole seems not varied,
it's fairly good agreement between the climatological and the magnetic evidence.
But it is not yet certain whether there has not been relative motion of the United States and of England,
which would make the situation more complicated.
Now I want to come, finally, to new experimental results to test out these theories.
Before I mention our Indian results, I want to mention those of Gough, of South Africa,
who studied the famous Pilanesberg dykes near Pretoria, which was known to be reversely magnetised a long time ago.
He discovered that this, we have the same diagram, that is, the magnetisation vertically downwards.
This is the South Magnetic Pole.
The South Magnetic Pole is in the southern hemisphere like mostly, coming up like that getting downward and to the point.
Instead of being magnetised in this direction, they're all magnetised in that direction,
and if you work out the present position of the South Pole from Gough's data, you find it is near Abyssinia.
Here is where the measurements were made, by Pretoria and the pole position, instead of being there, was there.
The result is extremely consistent and extremely reliable.
The only possible, reasonable explanation of this is,
South Africa has drifted right across the South Pole in the last 300 million years.
And just to show a slightly complicated direction relation, I will show you the following diagram.
I want to mention, these African rocks are all reversely magnetised.
So you have both, the reversal and the shift, to take into account.
Let us suppose that South African rock was, in fact, here is South Africa today,
this is a view, that's north, west, east, there's the South Pole.
We'll suppose that Africa has drifted across the pole like that.
If a rock was laid down, formed here, the South Pole normal direction of magnetisation would be up, like that.
And it would show the direction of magnetisation of the rocks if the earth's field had been normal at the time it was formed.
If it had been reversed, either chemically or really, then it would be that way.
Now suppose, this drifted across to here.
If it didn't lay down here, that would be the direction of magnetisation, or, to reverse, that direction.
But, in fact, it's that direction.
Which is exactly what this transforms to when you move around there.
So that the direction of the magnetisation of Gough's Pilanesberg dyke is consistent with the view
that Africa in the last 300 million years has moved right across the pole, roughly along a meridian.
Here you see some results flagged by Group's measurements in India.
We started on the Deccan Trap, which is the greatest outflow of horizontal lavas in the world, I think,
and is peculiarly suitable for our purpose, which is about 80 million years old.
And we found, we surveyed two sites, 400 miles apart, this is about half the results.
And these show a group of rocks, every disc is put here, the present earth field is shallow and pointing northward,
and these are steeply dipping, and in this direction.
This can be interpreted as meaning that all the rocks were reversed,
and that India has rotated anticlockwise by 24 degrees.
I should have said that Gough's South African results show a clockwise rotation of 20 degrees, but much older,
and that this angle, if you make the same diagram I did before,
corresponds to a latitude well south of the equator,
in fact, about 27 degrees south latitude compared with its present 25 degrees north latitude,
which comes out to be a rather faster shift than England,
in fact, about nearly three quarters of a degree per million years or some six metres a century.
Now, it is quite interesting that India comes from this independent magnetic measurement
to be half way between the equator and the latitude of 60 or so,
which would be required to produce the ice cap in Wegener's old diagram.
And as the ice age was 200 million years ago, this is roughly half way in time and space.
So, although it's extremely fragmentary evidence,
there is a slight indication of a steady northward movement for at least 200 millions years, of India,
and we've put it half way, and, of course, we are now trying to get extra points.
Recently, some work in Tasmania, by our group in conjunction with Jaeger, Dr. Jaeger in Canberra,
has shown that the dip of the Tasmanian rocks is very nearly vertical, whereas the present time it is that.
This is quite significant and suggests that Tasmania was quite near,
between the pole and its present position, 100 million years ago, this is Jurassic, 140 million years ago.
Comparing these with the Indian and the African results,
I think it's extremely likely that relative motion must have occurred as well as a general shift.
I would like to sum up this part by saying that I think the magnetic evidence,
for that the continents have shifted their position relative to the pole, is overwhelmingly strong.
I think some of the observations, a large part of them,
are consistent with at least a general movement of the crust relative to the pole,
but also some of the evidence seems to me increasingly pointing towards a marked relative motion
of bits of the land masses relative to each other.
So that polar wandering has certainly occurred and continental drift probably.
Now, just a last word on the more theoretical side.
A great amount of work has been done on theories of continental drift and of polar wandering
with completely contradictory results and leading to very little conviction in anybody's mind.
George Darwin did a great work on polar wandering 70 years ago,
in order to try and explain the Tertiary crises in ice ages by this means.
But the general conclusion was that you couldn't do it
and probably there had been neither polar wandering nor continental drift on dynamical grounds.
The matter has been recently taken up by Gold in Cambridge and by Munk, from America,
and they've concluded, rather independently, that polar wandering is a quite likely dynamical effect.
This brings us to consider what happens to the earth when, let's say the Himalayas or, to be nearer home, the Alps,
that is forcing up a large amount of metal, that will alter the dynamical condition of the rotating earth,
and the angular momentum axis will keep almost the same directional space,
but it can be shown or, rather, people think to know it can be shown,
because Darwin thought the opposite, George Darwin,
that the earth will change its crust's direction relative to the polar axis to accommodate this out of balance force.
It is mixed up with the plasticity of the earth,
the hypothesis is extremely complex and I don't pretend for a moment to follow the argument in detail.
But the experts consider that polar wandering is, in fact, a quite likely dynamical effect in contradiction to earlier view.
They, I think, have been influenced by the fact that we experimentalists have found it to occur, that often happens so.
Quite a separate point is how the continents can have drifted relative to each other,
and here there's a great deal of work being done, of a highly speculative character,
which is closely connected with the theory of mountain building.
And no one really knows, in the world, how mountains were built and why they're there.
But one of the speculations, again taken from Gough's admirable book, suggests a possible mechanism,
which is quite a useful one to think about.
Both, to explain why the Alps are here and why the continents may have drifted relative to each other,
if we on a way shown experimentally, show that they have.
And the theory is that the mantel of the earth,
which is usually considered as solid and which is, in fact, a solid as regards transmitting elastic earthquake waves,
is all the same plastic or semifluid for very long period disturbances.
We well know that in physics, material such as pitch, which bounds and yet, which sinks slowly under their own weight.
And the theory is, that there are some crustal currents in the mantel, not in the fluid, liquid core,
which are of thermal origin and which this is supposed to be an early stage of mountain building and continental drift.
Currents come up like this and drag the basaltic layer of rocks sideways.
This dotted region is what they call the sial, the continental litho rock, they drag these apart by viscosity,
so that gradually the basalt is forced in these directions and gradually dragged downwards, where it melts.
Under pressure, the continental block of sial is compressed here and gradually formed, pushed up.
As soon as the, while the current's flowing, the down thrust here,
there's a hollow and what the physicists, geophysicists call a geosyncline, in which a lot of the land is gradually sinking,
sediment gradually accumulate for millions of years, getting some 20 thousand feet or more thick of very light material.
Then the currents stop and isostasy reasserts itself, the iceberg rises and you get the mountain range.
That is about the current theory of mountain building and it links mountain building with continental drift.
The only doubt is a quantitative one.
Experts say that to make the Himalayas it would need a relative shift of this bit and that bit,
of up to, at most, 100 kilometres or so.
Some people say the Alps could be built by a shift of 20 kilometres, other say many hundred.
I think there's no doubt whatever that shifts of the order of 100 kilometres have occurred.
The problem is whether shifts of thousands of kilometres have occurred.
But this gives a fairly good account of, it's probably wrong, it's a very good model in the first instance.
Now, my last slide brings me nearer home, to a picture of the Alps.
And I do this, a geological picture of the Alps, not a postcard picture.
I do this for a very particular reason, and here I want to say something more general.
This rock magnetic work is undoubtedly going to have an enormous influence on a large part of geophysics in the future.
I may say that enormous amount of speculation and argument from the paleobotanists and paleobiologists
about these questions of continental drift and, or land bridges have gone on in the past,
so that any definite proof of what happened will have very big effect on a theory of many of the distributions
of animals, of flora and of a fauna of the world.
It will also have a profound effect on the geophysics of the earth's interior and on the theory of mountain building.
Now, I show this slide just to show that some of you, who live near these wonderful mountains,
have got a virgin experimental ground by tackling some of these geological problems magnetically in the Alps.
Roughly speaking, when the Alps were formed, the two land masses pushed together,
and these are sedimentary rocks, all laid down under the shallow sea in a geosyncline of falling land mass,
and then gradually pushed up again,
so that the high Alps are largely, but not entirely of sedimentary subocean or subsea origin.
Now, if one compares the magnetism of this part of the rock, if the rocks are stable,
you will be able to show that geologists really are right by saying that this outcrop here is the same as that one.
This being , of course, pure speculations.
So that, if you can find stable rocks,
you can check and deduce a great many of facts about the details of mountain building,
which can be only inferred indirectly by the geologist without this tool.
So that I suggest there's some very interesting work to be done on this subject.
Well, I've taken you a long way from where I started, the origin of cosmic rays,
but that is, in fact, how it happened, and I may say that we still don't know how the earth's magnetic field occurred.
And if the field reversals really did happen, that will be a straight jacket in which all future theories must fit.
And if you can tell the geophysicist that the earth's field reversed every million years
and flicked over like a multivibrator circuit, quickly, without intermediate theories,
he will have a guide on which to produce a decent theory.
But if, of course, it is not true, if the earth's field has remained in the same direction,
we already know that it hasn't changed appreciably in magnitude since the beginning of geological history,
then that is another fact that he will have to take into account.
(Applause)