62 NEW YORK STATE MUSEUM 



gneissoid somewhat altered forms which are commonly termed 

 greenstones. The characteristic green hue of these altered types 

 is due to the formation of a chloride mineral out of the hornblende 

 or biotite. 



Granodiorite is represented by the great area of so-called Har- 

 rison diorite in Westchester county and by smaller masses in both 

 the Adirondacks and southeastern New York. 



The physical characters of diorites are not very different from 

 those described under granites ; in the case of granodiorites the 

 resemblances are very close. They are a little darker in color, 

 never appearing in reddish tones, but always grayish or greenish ; 

 average around 2.8 or 2.9 in specific gravity, corresponding to a 

 mean of about 180 pounds to the cubic foot; and are useful for all 

 purposes to which granites are put, except they are less readily 

 polished, owing to the presence of so much hornblende and mica. 



GABBRO 



Gabbro is composed typically of pyroxene and plagioclase, the 

 latter being one of the more basic varieties — labradorite or anorthite. 

 Unlike the rocks previously described, it usually contains more of 

 the iron-bearing silicates than of feldspathic minerals and hence 

 the color is very dark, ranging from grayish or greenish gray to 

 black. The pyroxene includes both orthorhombic and monoclinic 

 varieties which very frequently show partial alteration to horn- 

 blende. Olivine is a common and at times an important ingredient ; 

 its presence is denoted by a prefix to the rock named, for example 

 olivine-gabbro. 



Gabbros are peculiarly subject to fluctuations in mineral com- 

 position through a relative gain in the proportion of one or another 

 of the common minerals, a variation caused by some process of 

 differentiation during the period of intrusion and consolidation. 

 By increase of the feldspar and corresponding shrinkage in the 

 pyroxene there results the rock already described as anorthosite. 

 This is really, therefore, a gabbroic type and not related directly to 

 syenite. The predominance of pyroxene leads to pyroxenite, in 

 which feldspar is very sparsely if at all represented. Olivine, with 

 subordinate amounts of feldspar and pyroxene, forms a peridotite. 

 The principal iron ore in gabbro is ilmenite which may be sufficiently 

 concentrated locally to form fairly pure masses of considerable 

 body. 



The gabbros, owing to their content of the iron-magnesia sili- 

 cates, are rather heavy, averaging from 2.8 to over 3 in specific 

 gravity. Their weight ranges from 175 to 200 pounds to the cubic 



