DIOKITES OP ELECTRIC PEAK. 99 



equally hot, or nearly so, indicating the previous heating by earlier magmas. 

 For convenience of description these rocks will be subdivided into the 

 following- groups : 



Ha. Varieties in which the dark-colored and the light-colored minerals 

 are in nearly the same proportions. 



lift. Varieties in which the light-colored minerals are in excess and 

 the amount of quartz is moderate. 



He. Like 116, but with much quartz. 



The dark-colored minerals include ferromagnesian minerals. The 

 others are feldspar and quartz. 



II a. VARIETIES IN WHICH THE DARK-COLORED AND LIGHT-COLORED MINERALS 



ARE NEARLY EQUAL. 



This group includes most of the stock rocks, and is the most basic. It 

 embraces a series of varieties that grade into one another chemically, 

 mineralogically, and structurally. 



As regards the degree of crystallization, or the size of the grain of the 

 rocks, they may be classed under 27 different grades, from fine to coarse. 

 But no attempt has been made to establish a scale of uniform degrees. 

 The arrangement is shown in Table VIII, Column Ila. At the coarsest- 

 grained end of the series are the diorites of the stock. Their structure is 

 hypidiomorphic granular. Some of the mineral constituents have their 

 proper crystallographic outline, but most of them are irregularly shaped. 



The constituents are lime-soda feldspars, hornblende, augite, hyper- 

 sthene, biotite, and quartz, with magnetite. The feldspars are more nearly 

 idiomorphic than the other constituents. They are rectangular to lath- 

 shaped, with outlines modified by the juxtaposition of other minerals. 

 Quartz forms cementing grains, wholly allotriomorphic. Hornblende, 

 pyroxene, and biotite seldom exhibit crystal boundaries, and penetrate one 

 another intricately. Magnetite is mostly found in the ferromagnesian sili- 

 cates. Apatite is colorless and in crudely formed crystals. Zircon is rare. 



The diorites of the seven highest grades of crystallization, Table VIII, 

 Column Ha, have a structure similar to that just given, but vary in the 

 relative abundance of the constituent minerals, as shown in Table V. In 

 the coarsest form the feldspars are from 2I> mm. to 1 mm. long, and the 

 quartz grains 0.25 mm. in diameter. The structure is shown in PI. XIX, 



