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18 



BEPORT — 1884. 



9. Pennsylvania hffore and after the FAt'catlon of the Appalachian Mom. 

 tains. Bij Professor £1. W. Clayi-olf, B.A., B.Se.Lond., F.G.S. 



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

 attempt to Iiandle, in a necessarily iniiHjvfect manner, and only to lirst approxima- 

 tions, a dinicult hut iniportiint and interesting peolo},'ical subject. Tiie uietiiDdof 

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

 the same purpose. 



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

 of the amount of compression or shortenin<,' produced at the siu-face by the corru- 

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

 to tiie American Atlantic .seaboard. 



In order to contine the paper within due limits, certain propositions must bi* 

 taken as proved. The principal of these are : 



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

 •J, 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 maimer in tlio 



later portion of tlu^ Palaeozoic era. 

 These admitted, the conclusion necessarily follows that during the formation 

 of the Appalachian Mountains a considerable contraction of the crumpled area 

 «iisued, 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 Mifldle Pennsylvania in plan 



and section with diagrams, and the field of study limited to eleven great raujj'es 



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



Pdue Mountain 

 Power Mountain 

 Conecocheagne .Mountaia 

 Tuscarora Mountain 

 AV. Shade Mountain 

 Black Loir Mountain 



Blue Ridge 

 Jack's Mountain 

 Standing Stone Mountain 

 Tussey Mountiiin 

 Bald Kagle Mountain 



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

 a-> given from near Warrior's Mark, in Huntingdon, to near (^arli.'^le, in Cumber- 

 land, sixty-ave miles in length. 



r'j. An attempt was made to estimate the length of the original contorted l)edof 

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

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

 bottoms of the .-^ynclines. and assuming for the ranges an average dip of 45', tlie 

 cnnclusion was reached that the lirst portion of the .section of forty-nine miles in 

 l-jngtli represented al)Oiit sixty miles of liorizontal stratum before it was cor- 

 rugated. The second portion being more strongly ])licated and its folds over- 

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

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

 trok place. 



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

 tlie ishoving of tlieir superficial .^strata over those below them, to an extent seldom 

 felly realised. The travelling of a whole county in this manner for tliirty or forty 

 niilos is a view in geology not easily pictured to the mind ; yet tlie soutb-e^st \m- 

 <;f Cumlwrland county must have moved over at least this distance. Toward tlif 

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

 airainst the beds of thi> Midland district, which formed the great buifer-plate oii 

 which the earth ])res«ure spent itself. 



In conclu.-^ion allusion was made to some .'suggestions which have Inyii put 

 forward to account for this crumpling, none of which are sufhcient, for gome cius' 

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

 brought forward. 



