426 DESCRIPTIONS OF ROCKS. 
oo per cent. or more of silica, and the basic usually less 
than 82. 
_ 6. The feldspars are divided, according to their bases, 
into (1) potash-feldspars, including orthoclase and miecro- 
cline; and (2) those which may be designated soda-lime- 
feldspars, namely the species albite, oligoclase, andesite, 
labradorite, and anorthite, which yield either soda. or lime. 
or both, on analysis. The term plagioclase has been used 
for the latter ; but it is no longer applicable since microcline 
is plagioclase. Under the heading potash-feldspars, as used 
beyond, leucite also is included; and under that of soda- 
lime-feldspars, nephelite and sodalite, and also the minerals 
of the saussurite group. 
The kinds of rocks are described under the heads of— 
1. FRAGMENTAL ROCKS, EXCLUSIVE OF LIMESTONES. 
2. LIMESTONES, OR CALCAREOUS ROCKS. 
3. CRYSTALLINE Rocks, EXCLUSIVE OF LIMESTONES. 
No strongly defined limit exists between the fragmental 
and crystalline rocks. But still they are for the most part 
widely diverse in character and aspect. 
In the names of rocks, the termination ite is here changed to yfe, as 
done in the author’s ‘‘ System of Mineralogy ” (1868), in order to dis- 
tinguish them from the names of minerals. Granite is excepted. 
I, Fragmental Rocks, exclusive of Limestones, 
1. Conglomerate.—A rock made up of pebbles or of coarse 
angular fragments of rocks of any kind. (qa) If the pebbles 
are rounded, the conglomerate is a pudding-stone ; (b) if 
angular, a breccia. 
Conglomerates are named according to their constituents, siliccous 
or quartzose, granitic, culeareous, porphyritic, pumiccous, etc. 
2. Grit.—A hard, gritty rock, consisting of coarse sand, 
or sand and small pebbles, called also mllstone grit, be- 
cause used sometimes for millstones. 
3. Sandstone.—A rock made from sand: a consolidated 
sand-bed. 
VARIETIES.—a. Siliceous or Quartzose ; consisting chiefly of quartz. 
b. Granitic; made of granitic material or comminuted granite. c. Mi- 
caceous ; containing much mica. d. Argilliceous ; containing much 
clay with thesand. e. Gritty ; hard and containing small quartz peb- 
