308 EVOLUTIONAL GEOLOGY. 



at an average depth of 2,000 fathoms below the land, and is main- 

 tained at a constant temperature, closely approaching 0° C, by the 

 passage over it of cold water creeping from the polar regions. The 

 average temperature of the surface of the land is above zero, but we 

 can afford to disregard the difference in temperature between it and the 

 ocean floor and ma}' 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 same depth as the ocean floor, a tempera- 

 ture of 87° C. will be reached on the supposition that the rate of increase 

 is 1° C. for 150 feet, while with the usuallj^ 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 l)e spherical, but will rise 

 or fall beneath an imaginary spherical or spheroidal surface according 

 as they occur IxMieath 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 

 40 or 50 miles. I'hus the subcontinental excess of temperature may 

 make itself felt in regions where the rocks still retain a high tempera- 

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

 The effect 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 sedhnent the sea floor sinks down, sub- 

 siding slowly, at about the same rate as the thickness of sediment 

 increases; and, whether as a consequence or a cause or both, the flex- 

 ure marking the boundary of land 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, possibl}' toward the land. At length when some 

 30,000 or 40,000 feet of sediment have accumulated in a basin-like 

 form, or, according to our reckoning, after the lapse of three or four 

 millions of j^ears, the downward movement ceases, and the mass of 

 sediment is subjected to powerful lateral compression, which, bringing 

 its borders into closer proximit}^ by some 10 or 30 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 contraction in con- 

 sequence, that the exterior, in adapting itself to the shrunken body, 

 was compelled to fit it like a wrinkled garment. This theory, indeed, 



