GEOLOGY OF MOUNT MARCY 37 



explanation would remove also certain difficulties met in other 

 exposures, which have been referred not unnaturally from the vari- 

 able mineralogy sometimes to the syenites, and sometimes to the 

 gabbros. In the exposures at the woolen mill locality in Elizabeth- 

 town there was some evidence of anorthosite tonguing into the dark, 

 supposed gabbro. This was a relation attributed to pressure effects, 

 when the dark rock was classed with the basic gabbros which are 

 later in age than the anorthosites. If, however, the dark rocks are 

 surviving inclusions of Grenville sedimentary gneisses impregnated 

 with matter from the anorthosites, the resemblance now to gabbros 

 and now to syenites might be reasonably explained, as would also the 

 apparent older age of the dark rocks. Exposures are as a rule lim- 

 ited. They run a short distance along a brook bottom, and then 

 disappear beneath the drift or forest growth. 



Besides the inclusions already mentioned there are often seen on 

 the mountains adjacent to the Keene valley and in the northern half 

 of the quadrangle, large masses of brown, rusty gneiss caught up in 

 the anorthosite. They vary from a foot or less across and 5 to lo 

 feet long, to other hundreds of feet in each diameter. Sometimes 

 rather pure quartz-orthoclase rock, they at other times (and espe- 

 cially the small ones) are rich in garnet and dark silicates. Much 

 uncertainty has been felt in their study as to whether they were 

 intrusive masses of syenites, as suggested by their weathering a 

 rusty brown ; or whether as is demonstrated by the small, angular 

 individuals, they are inclusions of some older rock in the anorthosite. 

 Difference of opinion might well arise in regard to the large masses, 

 whose peripheral relations are obscure. But even in the case of some 

 of these, the anorthosite appeared to cut in under them and to sup- 

 port the view that they are old inclusions of Grenville gneiss. They 

 appear on Baxter, Hopkins, Big Slide, Porter, Roosters Comb, 

 Gothics, and doubtless other peaks. An instance at the summit of 

 Giant mountain in the Elizabethtown sheet was interpreted in Bul- 

 letin 138 as a syenite dike. Additional experience has led, however, 

 to the interpretation here favored. 



In Bulletin 170, on the North Creek quadrangle. Prof. W. J. 

 Miller has mapped an unusual number of large and small intrusive 

 masses of basic gabbro and has applied to them very careful micro- 

 scopic and statistical mineralogical study, whose tabulation is given 

 on page 29. Expressed in percentages by volumes the values for 

 orthoclase are to those for oligoclase-labradorite as 32 :io, 32 :20, 

 50:15, and 45:15, showing in four out of the fourteen cases tabu- 



