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NEW YORK STATE MUSEUM 



Gabbro intrusions 



There are two main varieties of gabbroic rocks exposed in the 

 district: the one the coarse grained, blue-black rock, weathering 

 white, which forms the main mass of Eand hill; the other a fine- 

 grained, black rock, occurring only in dikes or in small bosses, 

 which cuts and is therefore later than the other — which is, in fact, 

 the youngest of the older Adirondack eruptives. It is one of the 

 most widespread rocks in the whole Adirondacks, though the 

 total bulk is not great. It is usually gneissoid, though often un- 

 changed cores remain, and these have an ophitic structure, that 

 is the feldspars are in well formed, lath-shaped crystals, giving 

 a characteristic appearance to the rock. It belongs to the gab- 

 bro variety known as hyperite, and has been frequently described 

 in detail, of late, by the different workers in the region, so that 

 no extended description is called for here. The gneissoid phase 

 is readily distinguished from the older, hornblende gneisses by 

 the usual dike-like form, finer grain, poorer foliation and general 

 more firm and resistant character. 



The first variety is merely a phase of an igneous rock which 

 has a wide extent in Essex and Franklin counties and whose 

 nearest outcrops are on Halleck hill north of Keeseville. The 

 Eand hill rock is peculiar and unlike any heretofore described 

 from the Adirondack region. While the ordinary Adirondack 

 rock is related to the gabbros, it is of a special and peculiar type 

 to which the name, anorthosite, has been applied. The Kand hill 

 rock is of an intermediate type which may be distinguished by 

 calling it anorthosite-gabbro. 



Anorthosite-gabbro 

 Anorthosite is a deep-seated igneous rock composed essentially 

 of labradorite (or some allied feldspar) with accessory augite, 

 hypersthene and titaniferous magnetite (or ilmenite), the total 

 amount of all these being but slight. Gabbro is a similar rock 

 of augite (or hypersthene) and labradorite in an approximation 

 to equal quantity. Anorthosite-gabbro, while predominantly 

 feldspathic, has the other minerals in sufficient quantity to make 



