176 REPORTS ON INVESTIGATIONS AND PROJECTS. 



would result. To give such protrusions as constitute the great pla- 

 teaus, the lateral compression of a segment having a depth propor- 

 tionately related to the extent of the plateau would seem to furnish 

 an appropriate agency, and hence the plateaus seem assignable to 

 rather deep-seated lateral movements competent to cause a widespread 

 upswelling of a large tract without excessive folding of its outer 

 surface. In this and the preceding cases reduction of volume by 

 compression, while recognized, is assumed to be a negligible factor. 

 (6) In the case of folded mountains, it soon becomes obvious by 

 numerical trial that the lateral thrust of a crust of an}' considerable 

 thickness, reduction of volume and downward deformation being 

 neglected, gives an amount of protrusion quite out of proportion to 

 the amount of folding and overthrust. For example, if a crust with a 

 thickness of 20 miles were thrust laterally 40 miles — Lesley's esti- 

 mate of the amount of the movement involved in the folding of the 

 ridges in central Pennsylvania west of Harrisburg — and gave rise to a 

 mountainous tract 60 miles wide, the average height would be 13 

 miles if the volume of the rock were essentially preserved by upfold- 

 ing, an amount quite out of accord with the facts. An effort to bring 

 the amount of lateral thrust and the amount of upward folding into 

 harmony with the actual phenomena led to the very important con- 

 clusion that the shell iiivolved in the thrust and folding is a very supo- 

 ftcial one. It is obvious upon consideration that in an earth assumed 

 to be solid throughout, whatever its origin, there should be a rather 

 close correspondence between the amount of protuberance involved 

 in the upfold and the amount of lateral thrust, for the very phe- 

 nomenon of outward folding implies that this is the direction of least 

 resistance for the operating stresses. Downward protrusion can 

 scarcely be assumed to be an important factor in this case — at least a 

 downward protrusion that does not by displacement at the same time 

 contribute to outward protrusion. While compression doubtless 

 results in some increase in the average density of the rocks affected, 

 neither theoretical calculations nor pendulum observations warrant 

 the belief that this constitutes an appreciable factor, and hence, as 

 indicated, the outward protrusion of the folded tract and the amount 

 of lateral thrust should be elements which mathematically combined 

 indicate the depth of the thrust shell. For example, if the thrust be 

 40 miles, the folded tract be 60 miles, the highest fold be 5 miles, 

 and the average elevation of the folded tract be 2.5 miles previous 

 to denudation, the depth of the thrust shell must be 3.75 miles, 

 neglecting changes of density, downthrust, etc. If the average ele- 

 vation of the folded surface were 3 miles, the shell must have been 4.5 

 miles thick. Now, these are the numerical factors involved in the 



