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NEW YORK STATE MUSEUM 



more likely are simply outliers of the main mass, connected with. 

 it not far underground. The present areal distribution of the rock 

 merely gives its extent in the plane in which the present erosion 

 surface cuts it. Since this surface is irregular, we have some 

 slight idea of the thickness of the mass, the higher mountain 

 peaks furnishing vertical sections of over 3000 feet. But the 

 amount which has already been removed by erosion can be but 

 vaguely estimated, and the extent of the mass in depth is wholly 

 problematic. Any estimates of the original bulk of the mass can 

 be nothing but pure conjectures, except that it can safely be said 

 that it was vast. 



Mineralogy. These rocks are composed mainly, and sometimes 

 wholly, of basic plagioclase feldspar, usually labradorite but some- 

 times bytownite or anorthite. They are eruptives of the gabbro 

 family extra rich in feldspar, which forms from 90$ to 100$ of the 

 whole rock throughout most of its extent. The minerals next in 

 abundance are augite and ilmenite (or titaniferous magnetite), 

 followed by hypersthene. Minute apatites are usually present. 

 In the many differing phases which the rock presents, several other 

 minerals creep in, the more common of which are hornblende, 

 biotite, garnet, microperthite, quartz, oligoclase and a sulfuret, 

 either pyrite, chalcopyrite or pyrrhotite. These are the original 

 minerals of the rock except where they are due to recrystallization 

 consequent on metamorphism. Subsequent alteration has locally 

 produced other minerals, notably zoisite, epidote, chlorite, scapo- 

 lite and muscovite, and surface decay has formed yet others. 



The feldspar is usually labradorite, twinning striations showing 

 plainly on fresh cleavage surfaces. The thin section usually shows 

 it to be full of minute, rodlike inclusions, all with parallel ar- 

 rangement, of some opaque mineral, likely ilmenite. These are 

 likely responsible for the usual dark blue color of the mineral, and 

 probably for the occasional iridescence, in greenish blue colors, as 

 well. This is by no means so frequent or so well displayed as in 

 the Labrador and Norway anorthosites, but is a common phenome- 

 non in the region. 



