294 Gushing — Igneous Bocks of the Adirondack Region. 



southern half of the region. In my judgment it is true that 

 the orthogneisses bulk more largely in the northern half of the 

 district, and the later intrusives in the southern. It is also 

 true that metamorphism is more excessive in the latter district, 

 so that the age relations of the intrusives are more masked 

 than on the north. I can simply record my disagreement 

 with his statement, and say that, in the northern Adirondacks, 

 the older rocks far exceed the younger ones in quantity, and 

 that, along the northern border, all across Clinton and Frank- 

 lin counties, they constitute the bulk of the Precambrian. 

 Here there is even very little Grenville ; nothing but a mono- 

 tonous repetition of granite gneiss and amphibolite, cut by 

 occasional small stocks of syenite and of gabbro. Topographic 

 maps still fail for most of this region, so that none of it has 

 been mapped in detail. But the above statement rests on 

 several years of reconnaissance work, which covered the whole 

 region. In St. Lawrence County more Grenville comes in and 

 separate bathylithic masses of the orthogneiss are readily 

 mapped. 



In so far as the possibility of separate mapping of the two 

 sets of intrusions over the whole Adirondacks is concerned, 

 that will very likely not be possible. The same difficulty is 

 met on a vastly larger scale in Canada. But that does not 

 alter the fact that at least two sets of intrusives are present 

 there, and that, in many places they can be told apart and suc- 

 cessfully mapped. 



Summary. — There are, in the Adirondacks, great bodies of 

 orthogneiss which have suffered severe regional metamorphism, 

 along with the Grenville rocks, and which have, in common 

 w T ith the Grenville, been invaded by the later intrusives of the 

 anorthosite-syenite group. The anorthosites cut out these 

 orthogneisses, and hold inclusions of them. The syenites 

 generally have granitic marginal phases, especially when in 

 contact w T ith the orthogneisses, but these later granites are 

 frequently found cutting the orthogneisses and holding inclu- 

 sions of them. The relations are especially clear in Jefferson 

 and St. Lawrence counties, where metamorphism is less severe 

 than on the southeast. Precisely the same relations obtain 

 across the river on the Canada side. It is a mistake to attempt 

 to class all these intrusives together, and to belittle the 

 importance of the early group. 



Western Reserve University, 

 Cleveland, 0. 



