PEGMATITE, SILEXITE, AND APLITE OF NEW YORK 51 



maline, the feldspars reaching lengths of a foot or more. Inclosing 

 both masses just mentioned is a body of coarse pegmatite, which 

 is the most abundant facies of the rock. The feldspar all seems 

 to be orthoclase in crystals, with maximum lengths of six or eight 

 inches. Quartz is abundant, and hornblende crystals up to several 

 inches long are fairly common. There are also exceptionally fine 

 graphic intergrowths of tourmahne and quartz in masses from a 

 few inches to a foot or more across (Fig. 6C). Along one side of 

 the coarse pegmatite there is a zone roughly five or six feet wide 

 of only moderately coarse pegmatite which consists chiefly of 

 potash feldspar and quartz with some acidic plagioclase and 

 biotite. Adjacent to this moderately coarse pegmatite and form- 

 ing the outside of the whole mass of pegmatite on one side is a 

 zone several feet wide of moderately coarse pegmatite made up 

 very largely of plagioclase which contains some microcline as 

 graphic intergrowths and a httle quartz. This plagioclase zone 

 merges into the true gabbro through a zone about a foot wide. 

 None of the five facies of pegmatite just described are at all sharply 

 separated from each other, and they are very clearly phases of 

 a single pegmatite body. The relation of the pegmatite to the 

 gabbro on the opposite side of the whole body of pegmatite is not 

 so well shown, but there appears to be a gradation within an inch 

 or two. On either side of the pegmatite body the gabbro is non- 

 foliated. Similar relations to the gabbro are exhibited where the 

 pegmatite body extends fifty feet northeastward, ending in broad 

 tongues in the gabbro. There, however, the plagioclase zone is 

 seven or eight feet wide, and it contains considerable quartz and 

 some biotite. One mass of gabbro five feet across is surrounded 

 by pegmatite with a fairly sharp contact on one side and a grada- 

 tion on the other. 



The assumptions that this whole body of pegmatite was a 

 distinctly later satelhtic development of the gabbro than that 

 above described as occurring on the opposite side of the same 

 stock, that the gabbro must have been almost or quite soHdified 

 before it formed, and that the five facies of the pegmatite were 

 probably formed by some sort of differentiation practically in the 

 place of their present occurrence are evidenced by the following 

 facts: gradation of the borders of the pegmatite into the gabbro, 



