62 C. II. Smyth. Jr. — Gahbros in the Adirondack Region. 



very sharp line of demarcation. As all the region is covered 

 with dense forest it is almost impossible to get any more defi- 

 nite knowledge of the relations of the different rocks. 



The black gneiss is coarse grained and usually more massive 

 than the surrounding rocks, though by no means always so. 

 Its most noticeable feature in the field is the presence of a 

 great deal of garnet, ranging from small specks up to lumps 

 two or three inches in diameter. Around the garnet there 

 may generally be seen a narrow rim of dark green or black 

 radiating plates. Freshly broken surfaces glisten with the 

 brilliant luster of numerous cleavage faces of hornblende. 

 From this normal type of the gneiss there are many variations. 

 By a decrease in ferro-magnesian minerals it becomes light 

 greenish-gray looking, very much like specimens of garnetifer- 

 ous anorthosite from Essex County. In other cases the garnet 

 disappears and the rock would then be taken for a basic gabbro. 

 Rapid changes in the character of the rock often take place 

 within the space of a few feet. 



Sections of the gneiss show a mineral composition closely 

 related to that of the gabbros. The feldspar is plagioclase, 

 but in most sections is almost entirely changed to minute color- 

 less scales, with the high double refraction, parallel extinction, 

 absorption, and negative optical character of muscovite. In 

 many sections this mineral wholly replaces plagioclase, but in 

 others portions of the feldspar remain showing every stage of 

 the alteration. Augite and hypersthene like that of the gab- 

 bro are abundant, together with hornblende of somewhat dif- 

 ferent character. This, though usually massive like the horn- 

 blende of the gabbro, differs from it in being pleochroic in 

 green tints, sometimes with a decided bluish tone. It is inti- 

 mately associated with both pyroxenes, and in such a way as to 

 suggest that it may be derived from them, though positive proof 

 of this is lacking. Magnetite and a little biotite make up the rest 

 of the rock, aside from the garnet. The latter, as already stated, 

 occurs abundantly in masses of varying size. It is sometimes 

 perfectly clear, with a decided pink color and sharply defined, 

 even boundary. In other cases its outline is very irregular 

 with embayments and inclusions of the other constituents, par- 

 ticularly the hornblende and pyroxene. 



The most conspicuous feature of the rock is its structure, 

 due to the peculiar form of the hypersthene and hornblende. 

 These two minerals, though often in irregular masses, show a 

 decided tendency to extend out in radiating tongues, with a 

 length often ten or twelve times their breadth. These tongues 

 may start from any portion of the section, but commonly 

 radiate from a mass of pyroxene or more particularly from 

 garnet when it is present, thus giving rise to the radiating 



