204 WILLIAM H. EMMONS 



stock. These vary in width from less than one inch to six or eight 

 inches, and contrast strongly in color and composition with the sur- 

 rounding rock. Basalt and andesite dikes which are exposed at 

 numerous places a short distance from its border do not cross the 

 stock, nor do they occur near its center. 



Minerals 0} the stock. — The constituent minerals of the stock, 

 though all are not present in a single specimen, in approximate order 

 of abundance are: plagioclase, orthoclase, augite, hypersthene, 

 biotite, quartz, hornblende, magnetite, olivine, apatite, pyrite, and 

 zircon. Plagioclase shows the usual albite and carlsbad twinning, 

 and frequently zonal structure. Some of the plagioclase contains 

 a large number of minute dark inclusions, which give it a grayish 

 color. Small prisms of apatite, together with a few anhedrons of 

 ferromagnesian minerals are similarly included. In composition the 

 plagioclase varies from oligoclase to labradorite. In the diorite and 

 quartz-diorite plagioclase is as a rule oligoclase or andesine, and in 

 the most basic types it is labradorite. The total amount of plagio- 

 clase in the different rocks of the stock is remarkably uniform, and 

 in most instances it constitutes from 40 per cent, to 50 per cent, of the 

 rock. Plagioclase was among the first minerals formed. 



Orthoclase, always present, and in some specimens in consider- 

 able amount, forms irregular anhedrons which fill the interstices 

 between all the other minerals, except quartz. Some of it incloses 

 ferromagnesian minerals, and apatite; and again it incloses plagio- 

 clase poikilitically or is microperthitic with albite. It was one of 

 the latest minerals formed, and constitutes from 2 to 18 per cent, of 

 the rock. Quartz occurs as irregular bodies in nearly all of the rocks 

 of the stock, though it is wanting in some of the olivine gabbros. 

 It reaches a maximum in the granodiorites, where it constitutes 

 nearly 20 per cent, of the rock. In many specimens quartz and 

 orthoclase form a micrographic intergrowth in which orthoclase 

 exceeds quartz. The micrographic intergrowth crystallized after 

 all other minerals had formed. 



Pyroxene, though not present in every facies of the stock is^ 

 altogether, the most abundant ferromagnesian mineral. Its anhedra 

 are for the most part irregular, but some of them approach idiomor- 

 phism. Augite and hypersthene are both present. Augite is pale violet. 



