l6 NEW YORK STATE MUSEUM 



biotite from brown to yellow. The labradorite is commonly filled 

 with myriads of tiny inclusions of a dark dustlike substance, prob- 

 ably ilmenite. 



Many of the hornblende gneisses and amphibolites of the quad- 

 rangle are demonstrably metagabbro because, in single areas and 

 even in single outcrops, they may be observed to grade into the true 

 gabbro. That most, or possibly all, of the bodies of gabbro and 

 metagabbro are older than the syenite-granite is proved by the fact 

 that they are more or less intricately cut by the granite and its peg- 

 matitic facies. In some cases, however, the relation of the gabbro 

 to the granite could not be definitely ascertained, so that possibly 

 some bodies of the gabbro may really be younger than the granite, 

 though quite certainly the amount of any such gabbro would be 

 small. It should be recalled that, throughout the Adirondacks, 

 many bodies of true gabbro have been considered to be younger 

 than the syenite-granite by several workers including Gushing,^ 

 Kemp,^ and the writer.^ Gradually, however, as these gabbros and 

 metagabbro facies have been more and more studied in their rela- 

 tion to the other rocks, it has become certain that many (or most) 

 of them are really older than the syenite-granite series. In fact it 

 now seems probable that any gabbros younger than the syenite- 

 granite series are far less conspicuously developed than the older 

 ones and, in the writer's opinion, it is even questioned if any at all 

 are of postsyenite-granite age. The evidence consists in finding 

 granite, and pegmatite dikes as offshoots of the granite, cutting the 

 gabbro and metagabbro at many localities. During the prosecution 

 of field work in the Lake Placid quadrangle in 191 5-16 the writer 

 first began to realize the probability of the presyenite-granite age of 

 at least some of the gabbro, and this idea became more and more 

 certain while mapping the Schroon Lake, Lyon Mountain, and Rus- 

 sell quadrangles, reports on two of which have been published." 

 Detailed evidence from the Luzerne quadrangle is presented below. 



There are many small areas, some of them large enough to be 

 separately mapped, of hornblende gneiss or amphibolite which are 

 not demonstrably metagabbro. These are in the form of lenses, 

 bands and irregular masses in the granite usually parallel to its 

 foliation. Similar rocks have been described by the writer as 

 occurring in various other quadrangles of the Adirondack region. 

 It is presumed that most or possibly all such masses are to be 



'N. Y. State Mus. Bui. 95, 1905, p. 330-31. 

 ' N. Y. State M'us. Bui. 138, 1910, p. 52-53. 

 * N. Y. State Mus. Bui. 170, 1914, p. 26-27. 

 "N. y. State Mus. Buls. 211, 213. 



