GEOLOGY OF MOUNT MARCY 2/ 



preference and the exposures have been assigned as far as possible 

 the syenitic color on the map. 



In the morainal deposit in the fields above the gulch of the brook, 

 literally scores of slabs of Potsdam sandstone can be observed. 

 They are flat and angular, just as if transported from a ledge a 

 short distance away. No Potsdam has ever been found in place for 

 many miles in all directions. 



The most reasonable interpretation of these observations on the 

 ores would seem to be one involving the intrusive entrance of both 

 the syenite and the anorthosite into the Grenville series and the 

 production of the garnet diopside . contact zones. Coincidently 

 came the development in several places of the bodies of magnetite, 

 which were large enough to have furnished in the old days of the 

 local forges commercial amounts of low phosphorus, low sulphur 

 iron ore. 



Contact on the west bank of the Ausable river. One and one- 

 half miles south of the north border of the quadrangle, and a short 

 distance north of the bridge where the main highway from Keene 

 valley to Elizabethtown crosses the river, is a very interesting, 

 coarsely crystalline contact rock consisting of deep red garnets, 

 black pyroxenes, which are a beautiful emerald green in thin sections, 

 calcite and scapolite. The rock is illustrated in color plate 8 and in 

 black and white plate 19A. The pyroxene is probably the variety 

 hedenbergite, because while visibly pleochroic, green to yellowish 

 green, it has a high extinction angle and can not be aegirite, as also 

 remarked by Max Roesler in a contribution, later embodied in this 

 bulletin. An extensive ledge of the garnet-pyroxene rock is exposed 

 next the highway. 



Summary of the Grenville Series 



The preceding descriptions of the Grenville series will make 

 clear that the ancient sediments are involved in a complicated way 

 with the later intruded anorthosites and syenitic rocks. The lime- 

 stones are our best preserved and recognizable originals but even 

 they have given rise to contact zones and have been replaced with 

 magnetite. The gneisses are chiefly microperthite-quartz rocks, 

 with some green diopside. They are much the same as some 

 acidic phases of the syenite series, but on the whole are believed to 

 be old Grenville beds which may have suffered changes from the 

 influence of the intrusives. At least three separate times the writer 

 has returned to study the outcrops, only to find them so complicated 



