FOLDING OF THE APPALACHIANS 903 



former geosynclines. Bailey Willis (44) has pointed out that 

 the gradual downward sinking of the strata in the geosyncline gives 

 them an individual steepness along the margin of the depression, 

 and that this initial dip forms the beginning of the deformation, 

 and determines the subsequent folding under the influence of lateral 

 pressure. The folds of the Appalachians are asymmetrical anti- 

 clines, with the steeply inclined, vertical, or even overturned limb 

 on the northwest or away from the present ocean. It must, how- 

 ever, be remembered that the present Atlantic coast is near the 

 western border of the old Appalachian land, and that hence if the 

 folds occurred before this land mass disappeared by subsidence, 

 the thrusting, which was from the east or southeast, was performed 

 or transmitted by this ancient land mass. In this westward thrust- 

 ing many folds were not only overturned, but thrust-faults of con- 

 siderable magnitude were developed. Thus the crystalline range 

 of the Hudson Plighland was thrust northwestward over the Palaeo- 

 zoic strata (Berkey) and numerous other overthrusts were devel- 

 oped in the Palaeozoic and older strata along the entire Appalachian 

 range. 



FoRESiioRTEis'iNG OF THE Crust. During the folding of the 

 Appalachians there was, of course, a considerable amount of fore- 

 shortening of the earth's crust. Lesley long ago estimated the 

 movement to have extended over a distance of forty miles, while 

 Claypole, considering the major folds, arrived at the approximate 

 conclusion that a section of the earth's surface measuring origin- 

 ally one hundred and fifty-three miles had been compressed into 

 sixty-five miles. (4.) 



Recently R. T. Chamberlin (2) has published the results of 

 some careful measurements of the Appalachian folds in the district 

 between Harrisburg and Tyrone, Pennsylvania, and these have fur- 

 nished more exact data for the estimation of the amount of crustal 

 foreshortening, and have likewise thrown some light on the form 

 and thickness of the crust involved in the folding. Pie concludes 

 that, so far as his measurements warranted, and postulating tlie 

 correctness of the assumed basis of investigation, the region west 

 of Harrisburg showed 81 miles of strata compressed into 66 miles. 

 The uncertainties of this result lie in the difticulty of eliminating 

 the amount of loss due to thickening and thinning of strata in 

 various parts of the fold; the variation in closeness of folding; or 

 the subsequent relaxation of the strata under the influence of 

 gravity, with resultant gliding on the limb of the fold. These im- 

 certainties can make such estimates only apiM-oximate. 



Height of the Folds. Assuming that, when the folding of 



