PRESIDENTIAL ADDRESS. 349 
indication of a difference between the physical nature of the sub-oceanic and 
sub-continental parts of the crust is in rough correspondence with the conclusions 
previously suggested on quite other grounds. 
In his Presidential Address to the Geographical Section of the British 
Association at Dover in 1899, the late Sir John Murray drew attention to the 
chemical differentiation which has been going on between the continents and 
the oceans since the processes of weathering and denudation commenced. By 
these processes the more siliceous and specifically lighter constituents are left 
behind on the continents, while the heavier bases are carried out to the ocean. 
It is to this process that Professor T. C. Chamberlin! also ascribes the origin 
of the depressions in which the oceanic waters have accumulated. As a 
corollary of the planetesimal theory, Chamberlin assumes that water began to 
be forced out of the porous surface blocks of the accumulated meteoritic material 
when the Earth’s radius was between 1,500 and 1,800 miles shorter than it is 
now; at that time pools of water began to be formed on the surface, and the 
atmosphere, just commencing its work, began the operation of leaching the 
heavier bases out of the highlands. Growth of the world proceeded by the 
infall of planetesimals, and while those meteorites that fell on the highlands 
became deprived of their soluble bases, those that fell into the young ocean 
were merely buried unaltered. Thus, by the time the Earth reached its present 
size its crust under the oceanic depressions must have developed a chemical 
composition differing from that under the continents. According to the deduc- 
tion suggested by Oldham from the seismographic records, there is a noticeable 
difference in the sub-oceanic areas to depths of between 1,000 and 1,300 miles—a 
layer in which the followers of Chamberlin’s theory might reasonably expect 
some physical expression of the partially developed chemical differentiation. 
The occurrence of denser material below the oceans has, of course, long 
been assumed from the deflection of the plumb-line, and was accepted by Pratt 
for his theory of compensation, as well as by Dutton as a wide expression of the 
theory of isostasy. Chamberlain '* thus explains the general prevalence of basic 
lavas in oceanic volcanoes. : 
The apparent heterogeneity indicated in the outer shell of the Earth to depths 
of 1,000 miles is naturally in conflict with the assumption that from thirty miles 
or so down the materials are in a liquid condition; at any rate, the idea con- 
flicts with Fisher’s extreme conception of the liquid substratum, in which the 
fluidity is supposed to be sufficient for the production of convection currents, 
upwards beneath the oceanic depressions, spreading horizontally towards the 
continents, and thence downwards to complete the circuit. 
The idea that changes of azimuth and of latitude may be brought about by 
the sliding of the Earth’s crust over its core has been put forward more than 
once to account for the climatic changes of past geological ages—the occurrence 
of temperate or even warm climates on parts of the crust now within the polar 
circles, and glacial conditions at the sea-level in countries like India, Australia, 
Africa, and South America, which are now far from the polar ice-sheets and 
in some cases near or within the tropics. Professor E. Koken, of Tiibingen,™ 
in an elaborate memoir entitled ‘Indisches Perm und die Permische Hiszeit,’ 
attributes the idea of a sliding crust to Mr. R. D. Oldham; but a similar 
suggestion was put forward by the late Sir John Evans twenty years before the 
publication of Mr. Oldham’s paper,’* and when the theory was restated in more 
precise form, ten years later,'® it was subjected to mathematical criticism by 
J. F. Twisden, E. Hill, and O. Fisher.*® 
1 Chamberlin and Salisbury, Geology, vol. ii. 1906, 106-111. 
12 Geology, ii. 1906, p. 120. 
SN. Jahrb. fiir Min. u.s.w., 1907, 537. 
™ J. Evans, ‘On a possible Geological Cause of Changes in the Position cf 
the Axis of the Earth’s Crust,’ Proc. Roy. Soc., xv. 46 (1866). 
** J. Evans, Presidential Address, Proc. Geol. Soc., 1876, p. 105. 
J. ¥. Twisden, ‘On possible Displacements of the Earth’s Axis of figure 
produced by Elevations and Depressions of her Surface,’ Quart. Journ. Geol. 
Soc., xxxiv. 35 (1877). E. Hill, ‘On the possibility of Changes in the Earth’s 
Axis,’ Geol. Mag., 1878, 262 and 479. 0. Fisher, ‘ On the possibility of Changes in 
the Latitude of Places on the Earth’s Surface,’ Geol. Mag., 1878, pp. 291 and 551- 
