to Section C {Geology). 459 



and the ocean floor, and may take them both at zero. Consider next 

 the increase of temperature with descent, which occurs beneath the 

 continents : at a depth of 13,000 feet, or at the same depth as the 

 ocean floor, a temperature of 87° C. will be reached on the supposition 

 that the rate of increase is 1° C. for 150 feet, while with the usually 

 accepted rate of 1° C. for 108 feet it would be 120° C. But at this 

 depth the ocean floor, which is on the same spherical surface, is at 

 0° C. Thus surfaces of equal temperature within the earth's crust 

 will not be spherical, but will rise or fall beneath an imaginary 

 spherical or spheroidal surface according as they occur beneath the 

 continents or the oceans. No doubt at some depth within the earth 

 the departure of isothermal surfaces from a spheroidal form will 

 disappear ; but considering the great breadth both of continents and 

 oceans this depth must be considerable, possibly even forty or fifty 

 miles. Thus the sub-continental excess of temperature may make 

 itself felt in regions where the rocks still retain a high temperature, 

 and are probably not far removed from the critical fusion-point. 

 The eifect will be to render the continents mobile as regards the 

 ocean floor; or, vice versa, the ocean floor will be stable compared 

 with the continental masses. Next it may be observed that the 

 continents pass into the bed of the ocean by a somewhat rapid 

 flexure, and that it is over this area of flexure that the sediments 

 denuded from the land are deposited. Under its load of sediment 

 the sea-floor sinks down, subsiding slowly, at about the same rate 

 as the thickness of sediment increases ; and, whether as a con- 

 sequence or a cause, or both, the flexure marking the boundary of 

 laud and sea becomes more pronounced. A compensating movement 

 occurs within the earth's crust, and solid material may flow from 

 under the subsiding area in the direction of least resistance, possibly 

 towards the land. At length, when some thirty or forty thousand 

 feet of sediment have accumulated in a basin-like form, or, according 

 to our reckoning, after the lapse of three or four millions of yeai'S, 

 the downward movement ceases, and the mass of sediment is sub- 

 jected to powerful lateral compression, which, bringing its borders 

 into closer proximity by some ten or thirty miles, causes it to rise 

 in great folds high into the air as a mountain chain. 



It is this last phase in the history of mountain-making which has 

 given geologists more cause for painful thought than probably any 

 other branch of their subject, not excluding even the age of the 

 earth. It was at first imagined that during the flow of time the 

 interior of the earth lost so much heat, and suffered so much con- 

 traction in consequence, that the exterior, in adapting itself to the 

 shrunken body, was compelled to fit it like a wrinkled garment. 

 This theory, indeed, enjoyed a happy existence till it fell into the 

 hands of mathematicians, when it fared very badly, and now lies in 

 a pitiable condition neglected by its friends.^ 



For it seemed proved to demonstration that the contraction con- 

 sequent on cooling was wholly, even ridiculously, inadequate to 



1 With some exceptions, notably Mr. C. Davison, a consistent supporter of the 

 theory of contraction. 



