718 report— 1884. 



9. Pennsylvania before and after the Elevation of the Appalachian Moun- 

 tains. By Professor E. W. Clatpole, B.A., B.Sc.Lond,, F.G.S. 



The paper, of which the following notes are an abstract, is intended as an 

 attempt to handle, in a necessarily imperfect manner, and only to first approxima- 

 tions, a difficult but important and interesting geological subject. The method of 

 treatment is, in the writer's opinion, one that has not hitherto been employed for 

 the same purpose. 



The object in view is to form some estimate, as near to the truth as possible, 

 of the amount of compression or shortening produced at the surface by the corru- 

 gation of the upper layers of the coast into mountain chains, with especial reference 

 to the American Atlantic seaboard. 



In order to confine the paper within due limits, certain propositions must be. 

 taken as proved. The principal of these are : 



1. That central contraction has developed tangential pressure in the crust. 



2. That the tangential pressure has produced crumpling of the crust. 



3. That to this crumpling are due long ranges of mountains. 



4. That the Appalachian Mountains came into being in this manner in the 



later portion of the Palaeozoic era. 

 These admitted, the conclusion necessarily follows that during the formation 

 ■of the Appalachian Mountains a considerable contraction of the crumpled area 

 •ensued, in a direction at right angles to that of the chain. 



The following points constitute the main features of the paper : 



1. A short account was given of the mountains of Middle Pennsylvania in plan 



and section with diagrams, and the field of study limited to eleven great ranges 



crossing the State from N.E. to S.W. These were 



Blue Mountain 

 Bower Mountain 

 Conecocheague Mountain 

 Tuscarora Mountain 

 W. Shade Mountain 

 Black Lopr Mountain 



Blue Ridge 

 Jack's Mountain 

 Standing Stone Mountain 

 Tussey Mountain 

 Bald Eagle Mountain 



2. A line of sections at right angles to these ranges was chosen and its position 

 as given from near Warrior's Mark, in Huntingdon, to near Carlisle, in Cumber- 

 land, sixty-five miles in length, 



3. An attempt was made to estimate the length of the original contorted bed of 

 Meania Sandstone of which all these mountains consist, aud after making ample 

 allowance for all necessary deductions for the flattened tops of the arches and 

 bottoms of the synclines, and assuming for the ranges an average dip of 45°, the 

 conclusion w r as reached that the first portion of the section of forty-nine miles in 

 length represented about sixty miles of horizontal stratum before it was cor- 

 rugated. The second portion being more strongly plicated and its folds over- 

 lapping gives yet higher results, and the whole sixty-five miles of the section line 

 were considered to represent about one hundred miles of surface before corrugation 

 took place. 



4. Such mass-motion as this involves the displacement of whole counties, and 

 the shoving of their superficial strata over those below them, to an extent seldom 

 fully realised. The travelling of a whole county in this manner for thirty or forty 

 miles is a view in geology not easily pictured to the mind ; yet the south-east line 

 of Cumberland county must have moved over at least this distance. Toward the 

 north-west this movement diminished, until the sliding, yielding mass was arrested 

 against the beds of the Midland district, which formed the great buffer-plate on 

 which the earth pressure spent itself. 



In conclusion allusion was made to some suggestions which have been put 

 forward to account for this crumpling, none of which are sufficient, for some cause 

 yielding a much larger amount of contraction is required to explain the facts here 

 brought forward. 



