588 GEOLOGY, 



the ''deeps " of the bottom of the ocean are notably concave. Fig. 455, 

 a cross-section of the Challenger Deep, drawn to true scale and con- 

 vexity, shows the nature of the phenomenon. The breadth is here 300 

 miles, and the depression below a true plane is 11,400 feet. The lower 

 line of the figure shows the approximate position and form of the normal 

 isogeotherm about ten miles below the surface. Assuming equal con- 

 ductivity in all parts, it is clear that the isogeotherms must be concave 

 upwards for a considerable distance below ten miles. Unless the shell 

 of thrust is much more than ten miles thick, these concave portions 

 should yield as fast as cooling below them permits, and no stresses 

 arising from convexity could he accumulated. 



These concavities of surface are so extensive and so widely dis- 

 tributed over the globe that no part of the outer shell can he supposed 

 to he capable of accumulating notable stresses unless rigidly attached to 

 the earth-body below. In other words, so far as sphericity is concerned, 

 the crust must ease allits stresses nearly as fast as they accumulate, if, 

 as usually assumed, it rests on a contracting or mobile substratum. 



Surface cooling under these conditions should give only feeble thrusts, 

 developed and eased nearly constantly. Such movements should be 

 admirably adapted to give those gentle, nearly constant subsidences that 

 furnish the nice adjustments of water-depth required for the accumula- 

 tion of thick strata in shallow water, and those slow upward warpings 

 that renew the feeding-grounds of erosion, the necessary complement of 

 the deposition. These gentle, nearly constant movements mark every 

 stage of geological history, and constitute one of its greatest though 

 least obtrusive features. But if superficial stresses arising in this way 

 are eased in producing these effects, they cannot accumulate to cause the 

 great periodic movements. 



Even where the crust is not concave, it is so warped and so traversed 

 by folds and fault-planes that its resistance to thrust is relatively low, 

 and it should, therefore, warp easily and at many points, if the thrust 

 be confined to a superficial crust. 



General conclusion. — When to the weakness of the crust, as com- 

 puted under ideal conditions, there is added the weakness inherent in 

 these concave and warped tracts, the conclusion seems imperative 

 that while the crust is the pliant subject of minor and nearly constant 

 warpings, such as are everywhere implied in the stratigraphic series, it 

 is wholly incompetent to be the medium of those great deformations 



