transition rocks 437 



Anorthosite and Syenite-granite transition Rocks 



(Keene Gneiss) 



general statements 



One of the most interesting rock types of the region is locally developed 

 as belts or irregular bodies along portions of the borders between the 

 anorthosite and the syenite-granite series. Both the Marcy and White- 

 face types of anorthosite show such border rocks. There is very strong 

 evidence, based on field-work and a study of thin sections, that this is 

 really a transition rock between anorthosite and syenite or granite due to 

 actual digestion or assimilation of anorthosite by the invading syenite- 

 granite magma along portions of its borders. It is here proposed that 

 this rock be called "Keene gneiss/' because a fine exposure of the typical 

 fresh rock occurs by the road just north of the village of Keene, in the 

 Lake Placid quadrangle. In view of the fact that many geologists main- 

 tain that there are no definitely proved cases of magmatic assimilation, 

 on considerable scales at least, the evidence furnished by these rocks, 

 especially in the Lake Placid and Schroon Lake quadrangles, has been 

 very carefully considered by me, and I am convinced that actual assimi- 

 lation has taken place. In other words, the Keene gneiss is quite cer- 

 tainly a good example of hybrid rocks, to use the term suggested by 

 Harker, who maintains that such rocks may be produced either by the 

 mixing of two distinct magma or by the assimilation of solid rock by a 

 magma. Fifteen areas of mostly Keene gneiss are represented on my 

 Lake Placid map. Several areas containing considerable developments 

 of Keene gneiss I have also found in the Schroon Lake quadrangle, one 

 of these occupying about 6 square miles and another about 3 square miles. 

 Others probably exist, but were not located owning to scarcity of outcrops 

 or roughness of country in some places. 



Gushing has described rocks, which I would put in the same category 

 with the Keene gneiss, from two localities on the western side of the great 

 anorthosite area. Kemp has described certain peculiar types of gabbro, 

 probably also to be classed as Keene gneiss, as occurring in the Elizabeth- 

 town quadrangle. Gushing suggests that these rocks in his districts are 

 magmatic assimilation products, but Kemp says nothing regarding the 

 origin of the peculiar types of gabbro in his district. So far as I know, 

 these are the only rocks of the sort regarding which even brief published 

 statements have been made. Obviously, there is great need of more data 

 regarding such rocks, because the problem of their origin and relations 

 to other rocks has a very important bearing on the wdiole problem of the 

 anorthosite. Since I have observed these rocks, often with the exhibition 



