106 STRUCTURAL AND FIELD GEOLOGY 
at the surface, were formerly overlaid by hundreds or even 
thousands of feet of younger strata which have since been 
removed by the gradual process of denudation, we cannot 
doubt that the mere weight of such enormous masses must 
have tended to consolidate the beds upon which they rested, 
Once more, we note that heat tends to solidify deposits, as 
may be seen in the case of strata which have been baked and 
hardened by intrusions of formerly molten matter—eruptive 
masses. More potent and widespread, however, must have 
been the action of the internal heat of the globe upon thick 
accumulations of sediment deposited during long-continued 
subsidence of the sea-floor. Certain consecutive series of 
strata attain a thickness of 15,000 or 20,000 feet and more. 
It is obvious, therefore, that while the upper members of such 
series were being accumulated, the lower members must have 
been more or less affected by the rise of the isogeotherms or 
lines of equal subterranean temperature. According to what 
is known of the increment of heat downwards, a very high 
temperature must obtain at depths of 15,000 or 20,000 feet 
from the surface—certainly much in excess of the boiling- 
point of water. Strata brought in this way under the 
influence of the internal heat of the globe could hardly escape 
some degree of change. Not only would they be compressed 
by the superincumbent masses, but if interstitial water were 
present, chemical reactions amongst the various rock-con- 
stituents might often be greatly stimulated—water acting as 
a more powerful agent under increased heat and pressure. It 
can hardly be doubted that such must have been among the 
chief causes of the consolidation of ancient sedimentary 
accumulations. 
Pressure may be brought about, however, by other means 
than the mere weight of overlying masses. The earth is a 
cooling body, and as the crust sinks down upon the slowly 
contracting nucleus, it necessarily becomes subject to enormous 
lateral compression. To this it can only yield by folding and 
crumpling up, and thus the rocks of which it is composed are 
frequently more or less highly disturbed. Strata which 
originally occupied approximately horizontal positions are 
now flexed, bent, and inclined at all angles, and such highly 
disturbed rocks, no matter what their geological age may be, 

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