348 E. B. MATHEWS AND W. J. MILLER — COCKEYSVILLE MARBLE 



suggested that it is possible to recognize in the highly crystalline and 

 much metamorphosed rocks of the area lines of bedding which indicate 

 that the crystalline rocks of the Piedmont possess a general structure 

 comparable to that of the Appalachians lying to the westward. In the 

 present paper it is proposed to give a somewhat more detailed discussion 

 of a local area, the special problem of the junior author, in which the 

 type of folding is well exhibited and more easily seen because of the 

 sharp differences, lithological and topographical, between the limestone 

 or marble and the adjacent rocks. 



The area under discussion occupies a tract of approximately 300 square 

 miles, represented on the southern portions of the Belair, Park ton, and 

 Westminster sheets and the northern part of the Gunpowder, Baltimore, 

 and Ellicott sheets of the United States Geological Survey, or, in other 

 words, between 76 degrees 25 minutes and 76 degrees 50 minutes west 

 longitude and 39 degrees 20 minutes and 39 degrees 40 minutes north 

 latitude. The Northern Central railroad from Baltimore to Harrisburg 

 passes directly across the region, while the Western Maryland from Balti- 

 more to Hanover skirts its western limits. 



Rocks of the Region 



list of the forma tions 



Within the limits of the region under discussion are exposed the rocks 

 of the four formations, namely, (1) Baltimore gneiss,* (2) Setters f quartz- 

 ite, (3) Cockeysville f marble, (4) Wissahickon * mica-schist, mica-gneiss, 

 and phyllite, described by the senior author in the discussions already 

 referred to. The lithological characters of these different formations vary 

 somewhat from place to place, but as exposed in the vicinity of the Cock- 

 e}^sville marble may be described as follows : 



BALTIMORE GNEISS 



This is a highly crystalline gneiss, composed of quartz, feldspar, and 

 mica, or hornblende, with accessory minerals so distributed as to pro- 

 duce a well marked gray banded gneiss, the individual bands of which 

 vary from a fraction of an inch upward. The average thickness, how- 

 ever, is quite small. Some of these beds are highly quartzose, resembling 

 a micaceous quartzite or less frequently a vitreous quartzite ; others are 

 rich in biotite or hornblende, producing dark to black rocks indistin- 

 guishable in a hand specimen from the mica and hornblende schists and 

 gneisses derived from igneous rocks by metamorphism. Through these 

 banded gneisses are intruded pegmatite and aplitic dikes more or less 



* Accepted by the Committee on Geologic Names of the U. S. Geological Survey. 

 fOld terms used provisionally until an agreement on names is reached. 



