912 PHYSIOGRAPHICAL GEOLOGY. [Boox VII. 
Geologists are now generally agreed that it is mainly to the 
effects of the secular contraction of our planet that the deformations 
and dislocations of the terrestrial crust are to be traced. The cool 
outer shell has sunk down upon the more rapidly contracting hot 
nucleus, and the enormous lateral compression thereby produced has 
thrown the crust into undulations, and even into the most complicated 
corrugations.’ Hence in the places where the crust has yielded to 
the pressure it must have been thickened, being folded or pushed 
over itself, or being thrown into double bulges, one portion of which 
rises into the air, while the corresponding portion descends into the 
interior. Mr. Fisher contends that this downward bulging of the 
lighter materials of the crust into a heavier substratum underneath 
the great mountain-uplifts of the surface is indicated by the observed 
diminution in the normal rate of augmentation of earth-temperature 
beneath mountains,’ and by the lessened deflection of the plumb-line 
in the same regions. 
The close connection between upheaval and denudation on the — 
one hand and depression and deposition on the other has often been 
remarked, and striking examples of it have been gathered from all 
parts of the world. Itis a familiar fact that along the central and 
highest parts of a mountain chain, the oldest strata have been laid 
bare after the removal of an enormous thickness of later deposits. 
The same region still remains high ground, even after prolonged 
denudation. Again, in areas where thick accumulations of sedi- 
mentary material have taken place there has always been contem- 
poraneous subsidence. So close and constant is this relationship as 
to have suggested the belief that denudation by unloading the 
crust allows it to rise, while deposition by loading it causes it to 
sink (ante, p. 287).° 
It is evident that in the results of terrestrial contraction on 
the surface of the whole planet, subsidence must always have been 
in excess of upheaval, that in fact upheaval has only occurred 
locally over areas where portions of the crust have been ridged up 
by the enormous tangential thrust of adjacent subsiding regions. 
The tracts which have thus been as it were squeezed out under the 
strain of contraction have been weaker parts of the crust and have 
usually been made use of again and again during geological time. 
They form the terrestrial regions of the earth’s surface. Thus, the 
continents as we now find them are the result of many successive 
uplifts, corresponding probably to concomitant depressions of the 
1 While these pages are passing through the press, the Rev. O. Fisher has published 
an able volume on the “ Physics of the Karth’s Crust,” in which he endeavours to show 
that the secular contraction of a solid globe through mere cooling will not account for 
the observed phenomena; and he re-states his argument for the existence of a fluid sub- 
stratum between the crust and the nucleus. See ante, p. 53. 
* Op. cit. chap. xii. The rate observed in the Mont Cenis and Mont St. Gothard | 
Tunnels was about 1° Fahr. for every 100 feet, or only about half the usual rate. 
* This belief has in recent years been forcibly urged by American geologists who 
have studied the structure of the Western Territories. See especially the reports of 
Mr. Clarence King, Major Powell, and Captain Dutton. 
i ae 
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tees BOW 
