The common gabbro type consists essentially of long prisms of plagioclase embedded in light reddish augite. This ophitic structure, upon which the distinction between diabase and gabbro depends, is well developed, but does not show in hand specimens, so that for field use the distinction is impracticable; ordinarily the term gabbro is applied to the coarse grained, and diabase to the medium and finer grained varieties. The plagioclase of the coarse grained specimens could not be identified, being entirely altered to a coarse saussurite in which the epidote was aggregated into large grains. The augite is almost equally changed to strongly pleochroic hornblende possessing green and blue green pleochroic tints. This hornblende is not a fibrous variety but forms compact individuals, hence in the specimens studied it could not be certain that some of it was not primary. In some cases it is further altered to chlorite. Reaction between plagioclase and pyroxene seems to have taken place, for chlorite occurs among the plagioclase decomposition products as well as those of the pyroxene. Next to these, ilmenite is the most abundant constituent, occurring as irregular masses largely altered to leucoxene in which the original ‘gridiron’ structure is distinct. Quartz is present in subordinate amounts usually in micrographic intergrowth with the plagioclase. Small, well defined hexagonal rods of apatite, titanite crystals, and rare zircons are also present. The structure and mineral constitution render the term quartz-diabase appropriate. Finer grained specimens secured from dikes were found to be much fresher than the coarser types and yielded more satisfactory thin sections; the ophitic structure is more pronounced, but the mineral composition is the same. The plagioclase laths of one section were determined optically to be an intermediate labradorite. Small flakes of biotite partially altered to chlorite occur accessorily.