Dec. 4, 1885. ] OJL [Frazer. 



GENERAL NOTES.— SKETCH ON THE GEOLOGY OF 

 YORK COUNTY, PENNSYLVANIA. 



By Persifor Frazer. 

 {Read before the American Philosophical Society, December 4, 1SS5.) 



The conditions which make York county soil productive, the study of its 

 geology interesting, and that geology itself varied, are due to effects of move- 

 ment in early geological time, which, compared with those which have 

 shaped our continent, are so small that they can hardly be represented upon 

 a geological map of the United States of ordinary size. Yet, in a rough 

 and general way, York county is a partial imitation, on a very small scale, 

 of the United States, inasmuch as, like that part of the American continent, 

 it consists of a belt of Archaean rocks in the north-west ; of another in the 

 south-east; and that its intermediate portions are made up of newer form- 

 ations containing fossils. Cavities in the limestone containing lignite 

 and fossil plants, the latter resembling that of the present day, are not 

 rare. These and possibly a marl in Carroll township near Dillsburp - , 

 which, however, has yielded no fossils, represent the latest geological 

 period ; and thus it may be said that of the five great divisions of 

 the rocks of our planet : viz, the "original" (?) or Archaean ; the "old 

 life " or Palaeozoic; the "middle life" or Mesozoic ; the "new life" or 

 *Cainozoic (including under this head the Quaternary and Recent), and 

 the Eruptive or igneous, each has a representative (or several of them) 

 within the confines of the county. If it were of interest or profit, the 

 analogy might be pushed a little farther to include the occurrence of the 

 igneous rocks in the north-west ; the broad belt of Mesozoic strata which 

 abuts upon the Archaean (but, in the case of the continent, also 

 upon numerous masses of new rocks which are scattered over a great 

 part of their junction) ; the contact of the Palaeozoic (Siluric in both 

 cases) on the south-east border of the Mesozoic and the contact on the 

 south-east of the latter formation with the Archaean. The last feature of 

 the United States' geology, which fails in the case of York county, is the 

 border line of New Life or Cainozoic rocks to the south-east of all the 

 above formations ; and even this might be supplied if the limits of the 

 county were pushed a comparatively short distance across Mason and 

 Dixon's line, and into the State of Maryland. But enough has been made 

 of this fancy, which is only introduced in order to fix more securely upon 

 the memory the fact that, geologically speaking, York county may be 

 considered to be a part of a great accidented plain of which the general 

 trend is east of north and west of south. Its valleys, or portions of them, 

 have successively formed the ocean bottoms of four or five different geo- 

 logical periods, probably extending from first to last over many millions of 

 years. 



To Rogers' names of "Primal" (or the beginning); "Auroral" (or 



* Written frequently Cenozoic. 



