332 E. B. MATHEWS — MARYLAND AND PENNSYLVANIA PIEDMONT 



Three smaller areas occur in the vicinity of Baltimore. Two of them 

 are portions of anticlinal domes which are either completely enclosed by 

 overlying sediments or cut off by faults and igneous rocks, while the 

 third, underlying the northwestern part of Baltimore city, is entirely sur- 

 rounded by gabbro and other igneous masses and is overlain in great 

 measure by the Coastal Plain deposits. 



Between Baltimore and Washington, along the boundary line between 

 Montgomery and Prince George's counties, occurs another area of Balti- 

 more gneiss, which up to the present has not been fully investigated by 

 the author. 



The rocks in each of these areas consist of highly crystalline gneisses 

 composed of quartz, feldspar, and mica, with accessory minerals, which 

 are so distributed as to produce well marked, gray banded gneisses, the 

 individual bands of which vary from a fraction of an inch upward, the 

 average thickness, however, being quite thin. Some of these beds are 

 highly quartzose, resembling a micaceous quartzite; others are rich in 

 biotite or hornblende, producing dark schists, which in a hand specimen 

 are indistinguishable from metamorphosed igneous masses. Within the 

 area of Baltimore gneiss are numerous small bodies of metamorphosed 

 granites and more basic igneous rocks, which have been intruded into 

 the gneiss and subsequently metamorphosed until they are practically 

 indistinguishable from it. The differences in character can now and 

 then be recognized, but it has not been found possible to carry the map- 

 ping of these small igneous intrusions from one exposure to another, 

 and frequently they are so small that they could not be represented on 

 maps of the scale (1 : 62,500) employed. 



SETTERS QUARTZITE 



The Setters quartzite occurs usually as a narrow rim on the flanks of 

 the areas of Baltimore gneiss, but is not continuous or always present. 

 Thus in the easternmost areas of Baltimore gneiss previously described 

 no Setters quartzite was recognized. It was found, however, skirting the 

 anticlinal dome of Baltimore gneiss along northern Baltimore county, 

 where it occurs as a single band on the eastern end of the dome and as a 

 series of somewhat parallel ridges on the northwestern slope of the anti- 

 cline, where the contact between the underlying gneiss and the quartzite 

 is near the present surface of the country. On the southern slopes of the 

 anticlinorium the quartzite seems to be lacking along the western half, 

 but is found in varying thickness along the eastern part of the anticline. 



The quartzite also occurs in the smaller anticline about a granite mass 

 in the vicinity of Warren, just south of the previously described anti- 

 clinal dome. 



