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



dome ; but the observations within the Baltimore gneiss in the eastern 

 half of the lenticular area, to the eastward of Monkton and Phoenix, are 

 entirely in accord with those of the western portion, and indicate that 

 the gneisses occur in numerous tightly pinched folds, possessing a com- 

 mon strike parallel to the major axis of the dome and steep dips, some 

 of the time to the north and some of the time to the south. On the 

 whole, the northerly dips predominate, indicating that the anticline is 

 somewhat overturned to the south. 



Numerous exposures of the gneiss are found along the Big and Little 

 Gunpowder rivers and their tributaries, but they are few and unsatis- 

 factory, except for areal mapping, in the plateau portions of the region 

 which is a prosperous farming community under a good state of cultiva- 

 tion. The exposures encountered show that the Baltimore gneiss is 

 penetrated in the northern area by igneous intrusions, now much meta- 

 morphosed, of granitic and gabbroic materials. The former are altered 

 to granite-gneisses and the latter to hornblende-schists. 



DISTRIBUTION OF SETTERS QUARTZITE 



General characteristics of the exposures. — This quartzite formation, with 

 its local variations toward a garnet-schist, occurs on the borders of the 

 Baltimore gneiss area, usually as long, narrow ridges, rising steepty 

 from the level of the limestone valleys to the surface of the plateau or 

 upland, and usually may be found outcropping where the streams have 

 cut across the strike. Its exposures are highly characteristic, though 

 generally poor, on account of the small rhomboidal form of the frag- 

 ments into which it breaks when fine bedded and somewhat micaceous. 

 When highly quartzose and more compact it is likely to be confused 

 with the Baltimore gneiss, and when very micaceous and carrying gar- 

 nets it is likely to be confused with the Wissahickon schists which overlie 

 the limestones. 



Southern area. — The ocurrence of the quartzite about the Baltimore 

 gneiss dome in the southwestern part of the area may be traced almost 

 continuously, by means of fragments, around the entire zone, and ex- 

 posures in which it is possible to obtain the dip and strike of the beds 

 may be found frequently. In this area, the typical area for this forma- 

 tion, the rock is characteristically thin-bedded, the beds being separated 

 by films of sericitic mica. Along the northern border of the anticline 

 from Rockland to Red Run the rock dips uniformly northward under- 

 neath the marble at an inclination of about 70 degrees. On the south 

 the dips are less uniform, being sometimes to the north, but generally 

 to the south, indicating a minor folding in the beds and some overturn- 



