HYPOTHETICAL STAGES LEADING UP TO THE KNOWN ERAS. 125 



ness of the distorted segment. The principles controlling the distribution of 

 strains is thought to require this. Thinner segments should give plateaus of less 

 breadth, and their heights should be determined by the extent of lateral thrust. 

 In such cases, the crumpling at the surface would be small relative to the whole 

 elevation. Some tension might even be developed on the top of the swell. 

 Plateaus are commonly affected by much tensional faulting, but this may be due 

 chiefly to the secondary action presently to be noted. They are also often affected 

 by the more superficial distortions of the crust yet to be considered. 



6. Under the hypothesis of a molten earth, it is agreed very generally that if 

 a solid crust formed over the interior while the latter was hot and expanded, and 

 that if the interior cooled and shrank later, the crust must have been too large for 

 its contracted core and must have wrinkled to accommodate itself to it, involving 

 horizontal thrust. If such a crust, whether formed in this or any other way, 

 having a thickness of 20 miles, were thrust laterally 40 miles — Lesley's estimate of 

 the amount of lateral movement involved in the folding of the Appalachian ridges 

 in central Pennsylvania J — and folded into a mountainous tract 60 miles wide, 

 and if the volume of the rock were preserved by upfolding, the average height of 

 the upfold would be 13 miles, an amount which is quite out of accord with the 

 facts. It is obvious on numerical inspection that no crustal thickness of this 

 order of magnitude can give rise to the actual foldings with the actual elevation, 

 if the approximate volume is preserved by upfolding simply. If preserved by 

 infolding, which would probably be the normal mode in a floating crust, the 

 result should probably be a synclinorium at the surface, with a very different 

 expression from that which is actually presented. If the crust were mashed so 

 as to give up-swelling in small part, and down-swelling in much larger part, a 

 closer approximation to the real case might be made, but even then it is doubtful 

 whether the configuration would be what it is. 



With an earth solid throughout, whether from accretion or otherwise, no such 

 disproportionate downward swelling is admissible. The thrust of a thick crust 

 is competent to form a plateau, with some crumpling, but apparently not the 

 requisite folding of the folded mountains with their limited elevations. 



7. All hypotheses that postulate a solid globe, and perhaps all others, must 

 apparently also postulate a shearing zone at a depth such that the amount of the 

 lateral thrust correlated with the thickness of the thrust shell, shall correspond 

 to the height of the folded tract, when suitable allowances are made for the com- 

 pression and extension of the rocks, perhaps for some downward displacement, 

 and for other incidental results, all of which are probably of small value. It is 

 of prime importance to determine what this thickness is. 



Ths thickness of the folded shell. — The thickness of the folded shell appears 

 to be deducible from the extent of the thrust, as shown by the folds, combined 

 with the average height and breadth of the folded tract. For example, if a thrust 

 of 40 miles was involved in the folding of the Appalachian ridges in central Penn- 

 sylvania, as Lesley estimates, and if the highest fold was arched about 5 miles, 1 



1 2d Geol. Surv., Penn. Summary, Final Rept,, 1903, p. 206, 



3 2d Geol. Surv., Penn., Pt. X, p. xxi, 1885. 



