MOVEMENTS AND DEFORMATIONS 359 



at 74 miles; 1 the shortening for the Appalachians in Pennsylvania, 

 not including the crystalline belt on the east, at 46 miles; 2 that of 

 the Laramide Range in British America at 25 miles, 3 and that of 

 the Coast Range in California at 9 to 12 miles. 4 



Though these estimates cannot be taken as measurements, they 

 are sufficiently close approximations to make it clear that the 

 amount of shortening of the shell involved in mountain folding is 

 large. These estimates represent only that shortening of the cir- 

 cumference effected at certain times and places; the whole short- 

 ening of a circumference involves the shortening implied by all the 

 transverse folds on a given great circle. Usually a great circle does 

 not cross more than one or two strongly folded tracts of the same 

 age, from which it is inferred that the shortening on each great circle 

 at any one time was largely concentrated in a few tracts running 

 at large angles to each other. If the folding of one of the main 

 mountain ranges be doubled, it may perhaps represent roughly 

 the shortening for the circle normal to it, for its own period of folding. 

 If one is disposed to minimize the amount of folding, the estimate 

 of the shortening may perhaps be put roundly at 50 miles on a cir- 

 cumference, for each of the great mountain-making periods; or, 

 if disposed to make the estimate large, the shortening may be put 

 at 100 miles, or even more. For the whole shortening since the 

 beginning of the Paleozoic, perhaps twice these amounts might 

 suffice, for while there have been several mountain-making periods, 

 perhaps no more than two or three are entitled to be put in the first 

 order. Assuming the circumferential shortening to have been 

 50 miles during a given great mountain-folding period, the appro- 

 priate radial shortening is 8 miles. For the more generous estim- 

 ate of 100 miles, it is 16 miles. If these estimates are doubled for 

 the whole of the Paleozoic and later eras, the radial shortening is 

 16 and 32 miles, respectively. The shortening for earlier eras can 

 hardly be estimated from present data. 



1 Heim, Mechanismus der Gebirgsbildung, p. 213. 



2 Claypole, Am. Nat., Vol. XIX, p. 257, 1885. 



3 McConnel, Geol. Surv. of Canada, p. 33 D, 1886. 



4 LeConte, Elements of Geology, 5th ed., p. 266. 



