1883.] 447 [Crosby. 



tance below the earth's surface. This stratum, however, in order 

 to answer perfectly all the purposes of the geologist, need not be 

 very extensive in the vertical direction. If we assign it a volume 

 equal to one-hundredth that of the earth, I think there are few 

 geologists who will not find it sufficient. Nor need the stratum 

 be absolutely continuous, though it must be nearly so. Nor, 

 again, do we require anything more than a very low degree of 

 plasticity. 



Babbage's theory that elevation and subsidence of the earth's 

 crust are due to the expansion and contraction of rocks by heat- 

 ing and cooling is an adequate explanation of some local and lim- 

 ited movements, in volcanic districts. But it is no explanation 

 at all of extensive movements affecting large areas. Let us ob- 

 serve the increase in temperature required in one hundred miles 

 in thickness of the earth's crust to lift the great plateau of cen- 

 tral Asia from thf> level of the sea to its present altitude of 

 16,000 feet — approximately, an expansion of three miles in one 

 hundred. Taking Colonel Totten's highest coefficient of expan- 

 sion — .000,009,532, or say .000,01 — then to produce an expan- 

 sion of .03 we must have an average increase of 3,000° Fah. The 

 increase, evidently, cannot be so large near the surface, and con- 

 sequently must be larger at considerable depths. But at a depth 

 of one hundred miles, according to Thomson, the crust has 

 nearly its original temperature, and cannot lose or gain heat in 

 any such fashion. Therefore, a smaller thickness must be heated 

 still higher. But at all depths not very near the surface we sup- 

 pose the rocks to have temperatures near their fusing points ; 

 and an increase of from 3,000° to 5,000°, without increase of pres- 

 sure, would surely fuse them. And, further, no explanation is 

 offered as to where this heat comes from or goes to. A clear 

 statement of this theory, as applied to really important instances 

 of elevation and subsidence, is sufficient to refute it. It breaks 

 down with its own weight. It also fails to explain local move- 

 ments which are paroxysmal. Babbage appreciated some of these 

 difficulties, and suggested that the elevation of extensive areas of 

 the sea-floor, where sediments are accumulating, could be ex- 

 plained by expansion due to rising of the isogeotherms. But this 

 would not account for subsidence any where, nor for the eleva- 

 tion of any old land-surface. Besides, it is a well established fact 



