54 
ries, etc.), down to the amorphous rocks. Thus, beginning with 
the granites, which have from 8o to 65 per cent, of silica, we pass 
among the coarse grained rocks to the diorites, which have between 
65 and 55 per cent., then to the gabbros and diabases, having 
usually more than 45 per cent., and end with the peridotites, 
having below 45 per cent. A corresponding series begins with 
the syenites, and ends with the nepheline rocks. 
Cases 1, and. part of 2. — First row, granite and its vari- 
eties, such as granitite, graphic granite, etc. These are rocks 
having quartz, alkaline feldspar, and one or more minerals of the 
mica, amphibole, or pyroxene groups as essential constituents. 
Second row, granite-porphyry, quartz-porphyry, vitrophyre, 
felsophyer, etc. Like the preceding in composition, but more or 
less porphyretically developed. 
Third row, rhyolite, nevadite, piwtice, obsidian, etc. These 
are amorphous volcanic rocks, having high percentages of silica, 
usually more than 70 per cent. 
Upper Part of Cases 2 and 3.— Polished slabs, chiefly 
gf'anite, with some 7narbles. 
Keniainder of Case 2, and Case 3.—Syenite-7tephelinite 
series. 
First Group.— First row. Sye?iite, minette, etc. Holocrystal- 
line rocks, having orthoclase and biotite as essential constituents. 
Second row. Trachytes. Tertiary eruptive rocks, character- 
ized by the predominance of an alkaline feldspar, usually sanidine, 
and freedom from quartz. An iron bearing mineral is also usu- 
ally present. 
Second Group.— First row. Nepheline ox elaeolite syenites, 
rocks comprised of nepheline, orthoclase, and usually a pyroxenie 
mineral and plagioclase feldspar. 
Second row. Phonolites, rocks consisting of an alkaline feld- 
spar, with minerals of the nepheline and leucite groups, and usu- 
ally a monoclinic augite. 
Third Group. — Tephrites and basanites, rocks having neph- 
eline or leucite and lime-soda feldspar, as essential constituents. 
They are usually porphyritic in structure, with a more or less 
amorphous ground mass. 
Fourth Group. — Kersantite, leucite basalt, leucitite, 7iephe- 
line basalt, and nephelinite . Rocks containing leucite or nephe- 
line in place of feldspar, and these usually associated with augite. 
