GEOLOGY OF FRANKLIN COUNTY 101 



crystals remaining. In general the extreme varieties are rare, most 

 •of the rock consisting of partially granulated material. There is 

 also considerable variation in the fineness of the granulation itself 

 in different parts of the rock, giving it a quite different appear- 

 ance. The granulated portion is commonly greenish or greenish 

 white in color, presenting a strong contrast to the blue black 

 ►crystals. 



As the peripheral portions of the anorthosite intrusions are ap- 

 proached, the dark silicate minerals become more abundant, so 

 that the rock, while still very feldspathic, can not be regarded as 

 wholly made up of that mineral. It occupies in fact, an inter- 

 mediate position between typical anorthosite and gabbro, in which 

 rock the augite plays as important a role as the labradorite, and 

 may be called anorthosite-gabbro. The passage from one rock 

 into the other is a gradual one, and has clearly resulted from a 

 slight differentiation in the original intrusion during the process 

 *of cooling. 



Along with this progressive change in composition often goes 

 one in structure, the rock becoming on the whole less coarsely 

 crystalline and usually somewhat gneissoid. The large labra- 

 dorite crystals decrease in size and are less frequent. 2sot infre- 

 quently this change is carried so far that a distinct, rather fine 

 grained gneiss results, of totally different appearance from the 

 original rock, the only clue to its origin being the possibility of 

 "tracing it through intermediate varieties into the undoubted 

 anorthosite, and the fact that it still contains occasional augen of 

 ^dark blue labradorite. This rock weathers much more readily 

 than does the anorthosite, and has a brown rusty look which 

 causes it to appear more weathered than is really the case. In it 

 -hypersthene is often more abundant than augite, which is seldom 

 the fact in the main mass of the intrusion, so that the rock is a 

 norite rather than a gabbro. Except for the labradorite augen, 

 which may ultimately disappear, these brown gneisses exactly 

 resemble gneisses which belong with the syenites, shortly to be 

 described, as well as certain gabbro gneisses which occur asso- 

 ciated with the Grenville limestones. When anorthosite and 



