312 NEW YORK STATE MUSEUM 



than a local peculiar phase of the general anorthosite mass, but, 

 sio far as the writer is aware, no definite -evidence has been 

 forthcoming concerning the time relationsi of the two rocks. 



The syenites. In many parts of the Adirondack region there 

 are found considerable areas of igneous rocks of greenish gray 

 color and fairly uniform character, which have considerable 

 resemblance to some phases of the anorthosite and were till 

 comparatively recently confounded with them. In their normal 

 phases they are readily recognized, but they show variation both 

 in composition and in degree of foliation, giving rise to varieties 

 from one or both of these causes which are difficult of recogni- 

 tion. Originally they possessed nothing like the coarsely 

 crystalline character of the anorthosites and hence, even where 

 least metamorphosed, the amount of granulated material is 

 very large, and the uncrushed feldspar remnants are infrequent 

 and of small size. Like- the anorthosite, they become finer 

 grained and more gneissoid near their borders, passing over into 

 granular gneisises; and these become intricately involved with 

 the bordering rocks, the whole forming a tangled complex which 

 is exceedingly difficult to unravel. 



Though grayish green on freshly fractured surfaces, .these 

 rocks undergo rapid color changes on exposure, so that the 

 normal color is onjy to be seen in recent rock cuts. On slight 

 exposure it changes to a more pronounced green, then passes 

 over to a yellowish or brownish green, and longer exposure 

 changes the whole mass to a rusty brown [pi. 2] . Even freshly 

 stripped, glaciated surfaces show the latter color, though in 

 them it is often only skin deep. In the majority of exposures 

 only the rusty brown rock can be collected, though residual 

 greien spots may often be noted. The cause of the color changes 

 is not manifest, thin sections of specimens of all the varieties 

 except the rusty brown showing all the constituent minerals in 

 perfectly fresh condition; and even the latter is often so fresh as 

 to show little alteration in any of the minerals except the 

 hypersthene. 



These rocks are predominantly feldspathic though not so 

 markedly so asi are the anorthoisites. Because of their original 

 finer grain, they are mostly quite gneissoid, and feldspar augen 



