ROCKS OF THE REGION 349 



parallel to the regular banding of the gneiss. Many of the broader bands 

 composed usually of hornblende-schists are probably of igneous origin, 

 as has recently been shown for similar rocks in New York by Professor 

 Julien. The banding of these gneisses has generally been regarded as 

 secondary, but the fact that the strike and dip of the bands coincide with 

 the strike and dip of the sedimentary rocks around the nose of folds and 

 on either flank makes it seem probable that they are very often, if not 

 always, indicative of original variations in sedimentation. 



SETTERS qUARTZITE 



This formation as developed in its type locality, Setters ridge, in the 

 southwest corner of the area under discussion, is a fine grained, some- 

 what saccharoidal, thin-bedded quartzite of white, cream, or light-brown 

 color. The beds are usually separated by thin films of sericite in tiny 

 glistening flakes. On the surface of these mica-covered planes fre- 

 quently occur black tourmaline crystals, which, as first shown by Wil- 

 liams, give evidence of movement along these planes. In other portions 

 of the area, especially in the vicinity of Butler and along the Gunpowder 

 river west of Glencoe and east of Phoenix, the rock becomes more vitreous 

 and less clearly bedded. This is especially true in the cutting of the 

 Gunpowder just below Warren, where the quartzite is exposed in a double- 

 topped anticline of moderate size. 



Resting on top of the more homogeneous and quartzo3e members occa- 

 sionally occur quartzite bands interbedded with micaceous layers, which 

 in the hand specimen appear to be garnet schists, practically indistin- 

 guishable from the Wissahickon schist, which occurs higher in the 

 column. At times the quartzitic layers become insignificant and the 

 whole mass looks like a Wissahickon schist. There are, however, minor 

 features which can not be put into words, but which may be recognized 

 during the progress of continuous mapping, which render the aspect of 

 the exposure and the character of the rock valuable as aids in mapping. 

 It was at first thought that this portion of the quartzite formation was 

 in reality a faulted-in representative of the Wissahickon, but detailed 

 mapping of critical areas, where the formations possess more marked 

 minor folding, show that a fault can not explain the development of this 

 local member of the quartzite formation. 



The Setters quartzite is generally found dipping at a steep angle, and 

 because of its resistance to weathering agencies it is often a topographic 

 feature which aids in determining its limits. 



COCKEYSVILLE MARBLE 



The carbonate rocks, which because of their more extensive exploita- 

 tion and peculiarities in weathering have been of especial service in 



