GEOLOGY OF THE NORTHERN ADIRONDACK REGION 



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10$ to 250 of minerals other than feldspar. Much of it is very 

 gneissoid and finely granular, with great development of garnet. 

 Elsewhere it becomes tolerably coarse with frequent, often very 

 large feldspars remaining. It varies very rapidly in character 

 from place to place, so that practically all varieties of the rock 

 may be collected within a small area. This, together with the 

 accessibility of the locality and the frequent exposures, make it a 

 magnificent collecting ground. 



The Rand hill rock departs somewhat from the ordinary type 

 and is a most interesting rock. It is mostly thoroughly gneissoid 

 and with no feldspar augen. these only appearing in quantity at 

 the northern edge of the exposures. Since this is the more likely 

 condition of the central part of the mass, and since further ex- 

 posures in this direction are cut off by the overlying Potsdam 

 sandstone, it is probable that the rock extends considerably far- 

 ther northward under the Potsdam covering. 



The most important difference between this rock and the usual 

 anorthosite gabbro consists in the constant presence of quartz in 

 considerable amount, forming from 5$ to 10$ of the rock. Another 

 difference is found in the comparatively large amount of apatite 

 present, the average of the rock holding from 3$ to 5$ of this 

 mineral and the amount not infrequently rising to 10$. Aside 

 from these the minerals are those of the usual anorthosite gabbro, 

 though the minerals other than labradorite form from 30$ to 35$ 

 of the rock. 



The primary minerals which have been noted in the rock are feld- 

 spar (usually labradorite), augite, quartz, hornblende, hypersthene, 

 apatite, ilmenite, zircon, pyrite, pyrrhotite and titanite. Secondary 

 minerals are garnet, hornblende, biotite and quartz. There are 

 other minerals present which have resulted from surface altera- 

 tion of the foregoing, but they are the usual decomposition 

 products formed under such circumstances. The order of crys- 

 tallization was the usual one, first the zircon and apatite followed 

 by the iron ores, then the hypersthene, augite and hornblende, 

 then the feldspar and finally the quartz. The periods of forma- 

 tion of the pyroxenes and feldspar largely overlap. Augite is 



