10 



Heport of the State Geologist. 



an amount considerably m excess of that of the pyroxene. Much of the 

 quartz present exists as inclusions in the feldspar and hornblende. 



These gneisses are much more distinctly foliated than the other varieties, 

 due to the concentration of their dark silicates along the planes of foliation. 

 The y pass into the others either by insensible gradation or by becoming finely 

 interbanded with them. They also seem to grade into the basic gabbros of 

 the region ; at least these latter present phases practically not to be distin- 

 guished from them. The possible relationship between the two forms is one of 

 the most pressing and puzzling problems here presented for solution. 



The gneiss series is thought to be of igneous origin and in part, at least, 

 of Archaean age, in the sense in which that term is used by the U. S. 

 Geological Survey. There are, however, certain difficulties in the way of this 

 view. 



The magnetite deposits of the region are in this series, and, in large part 

 at least, in the augite-gneisses, the deposits at Mineville, Essex county, 

 according to Kemp, being in such rocks, as are also those of Lyon mountain 

 and of Arnold and Palmer hills in Clinton county. 



II. The GrenviUe (Oswegatchie) Series. The term "Oswegatchie 

 series " v\ as proposed by Smyth to include the coarsely crystalline limestones 

 and associated rocks as exposed in St. Lawrence, Lewis and Jefferson counties.* 

 In the writer's opinion these rocks are so similar to those of the typical 

 Grenville series of Logan, and are separated from them by such a compara- 

 tively slight geographic distance that that' term might with perfect propriety 

 be utilized for the New York rocks. 



This series is very heterogeneous in character. It comprises quartzose 

 gneisses and schists, darker colored quartz-feldspar-biotite gneisses, dioritic 

 and gabbroic gneisses, and occssional bands of coarsely crystalline limestone. 

 Graphite is an abundant mineral. Pyrite is another, aiding by its decomposi- 

 tion in the production of the rusty, decomposed aspect which some of the beds 

 present in outcrop. Sillimanite and tremolite are frequently present as is also 

 garnet. The rocks are cut by later gabbros and granites, and are accompanied 

 by belts of gneiss similar to the older gneiss, which seem at times to be inter- 

 stratified with the other rocks, but concerning whose real relationships we are 

 in doubt. 



For the most part the gneisses of this series differ widely in appearance 

 from the older gneisses, and may be distinguished from them in the field 

 almost at a glance. A considerable portion seems to be unquestionably of 



♦ C. H. Smyth, Jr., Rep. St- K» eologist, N. Y., 1803. Vol. I, p. 4%. 



