November 23, 1900.] 



SCIENCE. 



791 



ing 0° C, by the passage over it of cold 

 vcater creeping from the polar regions. The 

 average temperature of the surface of the 

 land is above zero, but we can afford to dis- 

 regard the difference in temperature be- 

 tween it and the ocean floor, and may take 

 them both at zero. Consider next the in- 

 crease 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 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 sur- 

 face, according as they occur beneath the 

 continents or the oceans. ]S"o 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 proba- 

 bly 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 sediment the sea floor 

 sinks down, subsiding slowly, at about the 

 same rate as the thickness of sediment in- 

 creases ; and whether as a consequence or a 

 cause, or both, the flexure 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 subsid- 

 ing 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 years, 

 the downward movement ceases, and the 

 mass of sediment is subjected 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 geolo- 

 gists 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 contrac- 

 tion 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 exist- 

 ence till it fell into the hands of mathema- 

 ticians, when it fared very badly, and now 

 lies in a pitiable condition neglected of its 

 friends.* 



For it seemed proved to demonstration 

 that the contraction consequent on cooling 

 was wholly, even ridiculously, inadequate 

 to explain the wrinkling. But when we 

 summon up courage to inquire into the data 

 on which the mathematical arguments are 

 based, we find that they include several as- 

 sumptions, the truth of which is by no 

 means self-evident. Thus it has been as- 

 sumed that the rate at which the fusion 

 point rises with increased pressure is con- 

 stant, and follows the same law as is deduced 



* With some exceptions, notably Mr. C. Davison, 

 a consistent supporter of the theory of contraction. 



