C. H. Smyth, Jr. — GaUbros in the Adirondack Region. 57 



The mineral composition and structure are characteristic of 

 the gabbro family, but the variation in relative amounts of the 

 constituents in different masses makes it difficult to include 

 them all under one minor subdivision of the family. In the 

 majority of cases hypersthene is the prevailing ferro-magnesian 

 constituent, making the rock norite ; but in other examples 

 hypersthene is subordinate to monoclinic pyroxene, so that it 

 must be classed as hypersthene gabbro. Less often hornblende 

 exceeds the pyroxene in quantity, the rock thus approaching 

 diorite in composition. In a single specimen hornblende 

 entirely replaces pyroxene, the specimen being taken from 

 near the edge of a small patch having the normal composition 

 throughout most of its extent. In general these dioritic 

 phases are quite limited and appear to be more common 

 towards the contact with the gneiss. In view of this varia- 

 tion in composition the rock may be best treated under the 

 broad term gabbro, which will serve to include the important 

 varieties, hypersthene gabbro and norite. 



The least variable and most abundant constituent is the 

 feldspar. It is conspicuously clear and free from all traces of 

 the dust-like inclusion so common in the feldspar of the gab- 

 bros. The polysynthetic twinning of plagioclase is usually 

 present and the extinction angles on the lamellae, ranging from 

 22° to 28°, show that the feldspar is a basic bytownite. Un- 

 striated sections of feldspar are common, but the chemical 

 analysis of the rock indicates, by the small amount of K 2 0, 

 that little or no orthoclase can be present. Undulatory extinc- 

 tion is very pronounced in the feldspar, and is never lacking. 

 Further effects of pressure are seen in bending and breaking 

 of the lamellse and sometimes in considerable granulation. In 

 the most gneissoid specimens there is a limited alteration of 

 feldspar to muscovite. 



In perhaps two-thirds of the sections examined hypersthene 

 is next to feldspar in order of abundance. In very rare in- 

 stances it shows an approach to crystal form, but usually is 

 extremely irregular in outline. It is fresh and clear, and per- 

 fectly free from the plate-like inclusions so common in the 

 mineral. It shows the usual pleochroism quite strongly ; a, 

 pale red ; b, pale yellow or colorless ; c, light green ; with very 

 slight differences of absorption. It is distinguished from 

 monoclinic pyroxene by this pleochroism, by its parallel ex- 

 tinction, and by its rather low double refraction, together with 

 a high mean index. 



The monoclinic pyroxene is pale green, or, in very thin sec- 

 tions, colorless, and has no pleochroism. Like the feldspar 

 and hypersthene it contains no inclusions other than the older 

 magnetite. The pinacoidal parting and fibrous structure of 



