Marcu 24, 1923] 
NATURE 
Be 

Letters to the Editor. 
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The Wegener Hypothesis of Continental Drift. 
Tue chief value of the discussion on the Wegener 
hypothesis is that it has given rise to a reconsideration 
of the problems presented by the configuration and 
relations of the major features of the earth’s surface. 
The elaborate structure of theory built up by Dr. 
Wegener, and so effectively criticised by Mr. Lake (see 
Nature, February 17, p. 226), will have few, if any, 
thorough-going defenders in this country, but some of 
its leading features cannot be lightly dismissed. Mr. 
Crook (NATURE, February 24, p. 255) has recalled to 
our notice the suggestions put forward by Osmond 
Fisher, and later by W. R. Pickering, that the separa- 
tion of the moon from the earth, which Sir George 
Darwin believed to have resulted from tidal action, 
took place in the region now occupied by the Pacific ; 
that our satellite took with it three quarters of the 
earth’s crust, and that the remaining quarter, from 
which our continents trace their descent, has since 
oett up into fragments which have drifted apart over 
e heavier fluid magma below, leaving channels 
between them, the most important of which is now 
the Atlantic Ocean. Here we have an interesting 
approximation to certain of the assumptions of the 
Wegener hypothesis ; but both Osmond Fisher and 
Pickering, it will be noticed, considered the separation 
of the continents to be the result of a general drift 
towards the Pacific. In this they differ from Wegener, 
who attributes it to a varying lag of the earth’s crust 
relatively to its interior, so that one portion became 
separated from another. 
f the former view is well founded there should be 
a certain amount of symmetry about an equatorial 
diameter drawn to the centre of the Pacific from its 
antipodes in Africa. It is therefore interesting to 
note that Prof. Sollas in a communication to the 
Geological Society in 1903 (Q.J.G.S., vol. 59, pp. 184-8) 
declares that “an axis of terrestrial symmetry” 
“passes through the middle of Africa on the one side, 
and the Pacific Ocean on the other,’’ a depression in 
the Pacific corresponding to a dome in Africa. He 
is inclined to accept Osmond Fisher’s hypothesis, that 
the Pacific owes its origin to the birth of the moon, 
and suggests that the African dome represented an 
unsuccessful attempt on the opposite side of the world 
to give birth to a second satellite. This symmetry is, 
it is true, obscured by the east and west folding, which 
is such a frequent feature in the earth’s crust, and is 
attributed by Wegener to a drift away from the poles 
towards the equator, but is not improbably the result 
of a movement from the equatorial region to the poles 
due to the slowing down of the earth’s rotation and 
consequent decrease of the ellipticity of its figure and 
of the equatorial protuberance. 
Like Mr. Crook, Prof. Sollas follows Suess in believ- 
ing that the Atlantic owes its origin not to continental 
movement but to the foundering of the tract which it 
now occupies. My principal object in writing is to 
int out that the hypothesis of the drifting apart of 
Korth and South America from Europe and Africa is 
quite consistent with that of a subsidence and sub- 
mergence of a great part of the ocean area that now 
separates them; and that the latter is in fact the 
consequence of the former. 
The evidence, based on similarity of lithological 
characters and fossil contents of the rocks, that South 
NO. 2786, VOL. 111] 
America east of the Andes and the Falkland Islands 
were once in much closer proximity to Africa, is to 
my mind conclusive, and scarcely less is that of a 
former association of a great part of India and of 
Australia with Africa. There seems, too, every reason 
to believe that, although masked in places by other 
tendencies, there has been a general movement of the 
lighter continental crust or Sial from Africa towards 
the Pacific. This appears to be a drift from a region 
of a comparatively low gravity to one of higher 
gravity. 
As we have seen, Wegener and others believe that 
the earth’s crust lags behind the solid core as a result 
of tidal retardation, and that this lag varies from point 
to point. If this is the case, the folded mountains 
that have their roots deep in the earth’s interior will, 
no doubt, have a smaller lag than other portions of the 
earth's crust, and therefore, as Prof. Joly suggests, a 
relative movement from west to east. How far such 
movements are of importance it is at present im- 
possible to say. 
Whatever may be the ultimate causes of the relative 
movement of continents, they can only be effective 
when they operate in a direction in which the earth’s 
crust does not possess sufficient rigidity to oppose 
them. We can no longer suppose that there is a fluid 
substratum to the earth’s crust. It is probable that 
the solid crystallised zone below the oceans is usually 
fifteen to twenty miles in thickness, and at those 
depths the enormous pressure of 90 to 120 tons to the 
inch will give a comparatively high rigidity at the 
temperature of about 800° C. that may there prevail, 
even to an uncrystallised magma. Where, however, 
a great thickness of sediment has accumulated in the 
neighbourhood of a continent ona sea-bottom, it will— 
as Dana was the first to point out—by acting as a 
blanket, cause the temperature of the rocks beneath to 
rise and become less rigid, especially if they are basic 
in composition. In this rise of temperature, Prof. 
Joly believes radioactivity plays an important part. 
At the same time the area concerned will sink slowly 
to satisfy the requirements of isostasy, forming a 
trough parallel to the coast line. Ifa period of com- 
pression in a direction at right angles to the coast 
supervenes, the rocks will yield to it and the trough 
will be laterally compressed and deepened while the 
sedimentary accumulations are thrown into folds. In 
this way the land masses surrounding the Pacific have 
been enabled gradually to advance inwards from its 
circumference, their progress being marked by folded 
mountain ranges. Yet the Pacific, as a whole, apart 
from the marginal portions, being comparatively free 
from sedimentation, has preserved its rigidity and 
successfully resisted compression. 
On the other hand, there seems reason to believe 
that Africa is in the main the centre of a region of 
tension, due to the outward drift of continental 
masses in the circumstances already described. It is 
obvious that the separation caused by such a move- 
ment must involve a deficiency of material in the 
separating tract, and a loss of stability on the margin 
of the separated masses. Sometimes the blocks into 
which they are divided by jointing will fall forward 
one upon another like a succession of bricks and so 
give rise to a number of faults dipping away from the 
rift. Examples of this are seen in Skye and Caithness. 
More usually the slow subcrustal movement towards 
the line of fracture will carry the solid crust with it. 
The result will be a series of faults hading towards 
the region of tension and with a downthrow in that 
direction. In North Devon and Cornwall I have 
shown that there is evidence that there has been a 
general debacle of the rocks towards the west in Ter- 
tiary times. North-west and south-east faults occu- 
every few yards with a considerable hade to the south- 
