552 



REPORT OF NATIONAL MUSEUM, 1890. 



of a structureless mass are amorphous. Intermediate varieties, in part 

 crystalline and in part amorphous, those carrying crystals imbedded 

 in a glassy or felsitic base, are called hypocrystalline. 



According to their mode of occurrence the eruptive rocks are divided 

 here into two main groups. (1) The intrusive or plutonic rocks, and 

 (2) the effusive, or volcanic rocks.* These two groups are then sub- 

 divided according to their mineral and chemical composition. 



A. — Intrusive or tlutonic rocks. 



This group includes those rocks which, while in a molten condition, 

 were forced up or intruded between the older and overlying rocks in 

 the form of dikes, bosses, or intrusive sheets, and which never reached 

 the surface, but cooled and crystallized at such depths and under such 

 pressure as to become bolocrystalline. They are found as a rule only 

 among the older rocks, since it is only here that the erosion has been 

 sufficient for their exposure. It is to be noticed that they are, as a 

 rule, even to the unaided eye, distinctly crystalline ; that they are 

 never amygdaloidal or vesicular, and very rarely show flow structure ; 

 all of which are features common to the effusive rocks next to be de- 

 scribed. 



According to their mineral and chemical composition the rocks of 

 this group are divided into nine subgroups, or families, as follows: (1) 

 The granites, (2) the syenites, (3) the nepheline syenites, (4) the di- 

 orites, (5) the gabbros, (6) the diabases, (7) the theralites, (8) the peri- 

 dotites, and (9) the pyroxenites. 



1. The granites. 



Granite, from the Latin granum, a grain in allusion to the granular 

 structure. 



Mineral composition.— The essential constituents of granite are quartz 

 and a potash, feldspar (either orthoclase or microcline), and plagio- 

 clase. Nearly always one or more minerals of the mica, hornblende, or 

 pyroxene group are present, and in small, usually microscopic forms, 

 the accessories magnetite, apatite, and zircon ; more rarely occur 

 sphene, beryl, topaz, tourmaline, garnet, epidote, allanite, fluorite, and 

 pyrite. Delesse* has made the following determination of the relative 

 proportion of the various constituents in two well-known granites : 



Egyptian red granite. 



Parts. 



Porphyritic granite, Vosges. 



Parts. 



Red orthoclase 

 White i.lbite .. 



Gray quartz .. 

 Black mica 



Total.— 



White orthoclase . 

 Reddish oligoclase 



Gray quartz 



Mica 



Total 



* Rosenbusch's group of dike rocks or gangesteine is here included with the plntouics. 

 t Prestwich Chemical and Physical Geology, vol. I, p. 42. 



