SETTERS QUARTZITE 333 



Quartzite from the typical ridge originally described by Professor 

 Williams occurs as a continuous belt about the small anticlinal dome 

 lying 10 miles northwest of Baltimore between the Northern Central and 

 Western Maryland railroads. The formation here is of rather uniform 

 thickness and stands at a steep, sometimes overturned, angle on the 

 flanks of the gneiss. Across the valley of lake Roland is a similar, 

 though less well defined, anticlinal arch of more complicated structure^ 

 along the sides of which may be seen the Setters quartzite, extending 

 from the Northern Central railroad on the west to the eastward until it 

 is cut off by overlying formations or igneous rocks. 



The Baltimore area of gneiss is included entirely within the gabbro, 

 and no quartzite is to be noted along its borders. 



On the western side of the Baltimore area, extending southward 

 through Howard county, may be found occasional exposures of Setters 

 quartzite between the gneiss and the marble. The work in this area has 

 not been completed, but it seems quite probable that the quartzite will 

 be found developed as a more or less continuous stratum lying between 

 the Baltimore gneiss on the east and the overlying marble on the west, 

 as it has already been recognized in this position at many points. 



The quartzite is a fine-grained, somewhat saccharoidal, thin-bedded 

 rock of white or cream color in its typical development along Setters 

 ridge. At this point the beds are usually separated by thin films of 

 muscovite or sericite in small sparkling flakes. On the surface between 

 the individual beds are black tourmalines, which have been more or less 

 disturbed, as shown by the stretching which they have undergone. The 

 Setters quartzite as a formation is, however, somewhat more variable than 

 was at first supposed from the study of the original locality on the south 

 side of Greenspring valley. Locally the rock may become very vitreous 

 and massive, resembling some of the Huronian quartzites of the Lake 

 Superior region. At other times the rock becomes more argillaceous, 

 with a development of garnets, staurolite, and other accessory minerals. 

 The development of such minerals causes the quartzite formation to 

 simulate in lithologic character the overlying Wissahickon mica-schist, 

 and at first occasioned considerable difficulty. It is now known, how- 

 ever, that the more quartzose layers may be intimately interbedded 

 with the more micaceous and garnetiferous ones toward the center of 

 the formation, and that the upper portion of the formation when well 

 developed may be highly micaceous and garnetiferous* The develop- 



* The presence of a somewhat sericitic quartzitic lower member and a more argillaceous over- 

 lying member is strikingly in accord with the characteristic Cambrian deposits of York county, 

 Pennsylvania, and those of the Oatoetin area described by Keith, as well as those of Balcony 

 falls, Virginia, as described by Campbell. There is, however, more metamorphism with the de- 

 velopment of metamorphic minerals in the rocks of the eastern Piedmont. 



