THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Aet. XXVII. — The Middle Ordovician of Central and 

 South Central Pennsylvania; by Richabd M. Field. 



Introduction. 



The geographic position, as well as the faunal and 

 lithological peculiarities of the Ordovician formations of 

 central and south-central Pennsylvania, make it possible 

 to treat their area, for descriptive purposes, as a part of 

 a province within the Ordovician terranes of eastern 

 North America. The sketch-map (fig. 1), copied from 

 the atlas of the Second Geological Survey of Pennsylva- 

 nia (l), 1 gives an approximate idea of the location and 

 distribution of the formations which are exposed in 

 eroded anticlinal valleys parallel to the northeast and 

 southwest axis of the Appalachians. The map shows all 

 of the formations from the Beekmantown to the Eden, no 

 attempt having been made to plot the exact areal distri- 

 bution of the Middle Ordovician formations alone. At 

 the time of the Second Geological Survey, all of the lime- 

 stone above the Upper Cambrian and below the Upper 

 Ordovician was represented by a single color, and called 

 the " Valley limestone. " The white numbers on the 

 black areas of the sketch-map refer to the principal sec- 

 tions in the Middle Ordovician which have been studied 

 by the writer and which will be described in detail else- 

 where. 



The large western area exposed in Bedford, Blair, Cen- 

 ter, Clinton and Lycoming Counties contains approxi- 

 mately 840 square miles, the northeastern section at 

 Salona being 105 miles from the Willow Grove section to 

 the northwest. In Center County the Ordovician lime- 



1 Eef erence numbers in parentheses in the text apply to the bibliography 

 at the end of the article. 



Am. Jour. Sci. — Fourth Series, Vol. XLVIII, No. 288. — December, 1919. 



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