AGE OF THE FORMATIONS 330 



the Wissahickon is itself of Ordovician age, since there is no well defined 

 and extensive line of separation between the Wissahickon and these 

 locally developed slates. 



Age of the Formations 



Within the Maryland Piedmont, so far as studied in detail, there are 

 no means of locating accurately the position in the stratigraphic column 

 of the four formations — Baltimore gneiss, Setters quartzite, Cockeysville 

 marble, and Wissahickon schists and shales — but on either end of a con- 

 tinuous extension of these formations we have a sequence into sediment- 

 ary rocks of known horizon. On the east Doctor Bascom has shown 

 that the Baltimore gneiss is pre-Cambrian, the quartzite Cambrian, the 

 limestone Cambro-Ordovician, and the Wissahickon Ordovician. On 

 the west the phyllites are apparently in the same relation to the known 

 Cambrian and Cambro-Silurian deposits, although in this area the geo- 

 logical mapping has not been conducted in detail on account of the lack 

 of satisfactory topographic maps. The conclusion from the facts at hand 

 would therefore seem to warrant the correlation shown on page 340. 



Opposed to this conclusion is the fact that directly on the strike of the 

 \Y r issahickon schist are a series of more quartzose and more compact 

 metamorphosed sediments, which have been regarded by Mr Keith, in 

 his discussion of the geology of the Washington quadrangle, as Carolina 

 gneiss, which, as defined by him, is pre-Cambrian. The grounds for the 

 assigning of such an age to rocks of this region involve the areal work 

 of Mr Keith in the region to the north and west of Washington, which 

 has not yet been published on account of a lack of a proper topographic 

 * 



Structure and Structural Relations of the Piedmont Formations 



For a proper understanding of the structural relations of the Piedmont 

 deposits of Maryland it is necessary to recognize the character of the 

 major structures along the eastern Atlantic coast from New Jersey south- 

 ward and the position of the Maryland deposits with respect to these 

 structures. The facts which are given below are familiar to students of 

 American geology, but it seems desirable to restate them in relation to 

 the Piedmont rocks under discussion. 



Among the more striking features of the continental structure along 



*A conference has been proposed for the workers in the Piedmont of Virginia, Maryland, Penn- 

 sylvania, and New Jersey, and an attempt will be made to reach a mutually satisfactory under- 

 standing as to the age correlations. Until this is done it is better to regard the age of the Maryland 

 deposits as unsettled, with the evidence favoring the position taken in the preceding^discnssion. 



