12 NEW YORK STATE MUSEUM 



with the anorthosites. From relationships which have been best 

 worked out in the Long Lake quadrangle by Professor Gushing^ and 

 which show the syenites to be later, the same relation is believed 

 to hold good in the Mount Marcy. The intermingling with anor- 

 thosites is complicated as will be later set forth. 



The anorthosites cover far the largest portion of the quadrangle. 

 Practically all the southern four-fifths consist of them. They are 

 somewhat variable in composition, the dark silicates mounting up 

 at times beyond the characteristic percentages of the typical anor- 

 thosite. There were presumably two outbreaks ; there may have 

 been more. The anorthosites appear in large, massive exposures 

 for the most part, but they also break through the Grenville sedi- 

 ments and include fragments of the latter in a very complex way. 

 They and the syenites have brought about some remarkable 

 contact zones with the Grenville limestones. One of these contains 

 deposits of iron ores. 



The Grenville series of sedimentary gneisses, limestones and rarer 

 quartzites embraces the oldest rocks of all. They are limited so far 

 as known to the northern portion of the Keene valley and the 

 adjacent mountains. They constitute a thick series of east and 

 west strike and steep dip. They are very ancient rocks and were 

 metamorphosed before the intrusion of the anorthosite. 



These several groups will now be described in order beginning 

 with the oldest. 



THE GRENVILLE SERIES 



The oldest rocks in the Mount Marcy quadrangle are a series of 

 sedimentary gneisses and crystalline limestones to which it is now 

 customary to apply the name Grenville. The name is adapted from 

 Canadian usage where it was first given by Sir William Logan to 

 strata of the above-mentioned character in the township of Gren- 

 ville, Ontario. The name has been generally adopted in the Adiron- 

 dack work, where the strata are now known to outcrop in almost 

 every quadrangle. Their most extensive development is in areas 

 away from the central eruptive masses and the completest description 

 thus far issued is by Prof. H. P. Gushing^ in his description of the 

 region of the Thousand Islands. 



The Grenville series occupies a relatively small portion of the 

 Mount Marcy quadrangle. Its best development is in the valley 

 of the East branch of the Ausable river, for about 2 miles upstream, 



^ N. Y. State Museum Bui. 115, p. 481, 1907. 

 2 N. Y. State Museum Bui. 145. 



