52 L1TH0L0GICAL GEOLOGY. 



of carbonate of lime that have been produced by direct chemical deposition from the 

 waters of the globe are small compared with those made of organic remains of plants 

 or animals. 



The nine elements above mentioned, oxygen, silicon, aluminum, magnesium, calcium, 

 potassium, sodium, iron, and carbon, are the prominent constituents of rocks, making 

 up 977-1000ths of the whole. 



(10.) Sulphur. — Sulphur exists native in volcanic and some other regions. In com- 

 bination with various minerals, it forms ores called sulphids, as sulphid of iron, or pyrite. 

 sulphid of copper, sulphid of silver. But these sulphids do not constitute properly 

 beds of rock; although two of them, pyrite and pyrrhotite, are very abundant. Sulphur 

 forms with oxygen two acids, sulphurous acid (SO-), and sulphuric acid (SO 3 ). Sul- 

 phuric acid united with lime makes sulphate of lime, or gypsum, which sometimes 

 occurs in extensive beds. There are also many other sulphates, but none are true rock- 

 constituents. 



(11.) Hydrogen with oxygen constitutes water; and water, besides being abundant 

 over the earth's surface, is a constituent of many minerals. Gypsum contains 21 per 

 cent., serpentine 13 per cent., talc 5 per cent. 



(12.) Chlorine with sodium forms chlorid of sodium, or common salt, which is found 

 in large beds, and also dissolved in sea-water and brine-springs. 



(13.) Nitrogen is an ingredient of the atmosphere, — making 77 per cent, of it. With 

 oxygen it forms nitric acid (NO 5 ); but no nitrates enter prominently into the structure 

 of rocks. 



The thirteen elements mentioned are all that occur as important 

 rock-constituents. Others require attention in discussing topics con- 

 nected with chemical geology, in which department the profoundest 

 knowledge of chemistry and mineralogy is none too much. But in a 

 general review of rocks only these thirteen need be considered. 



2. MINERALS CONSTITUTING ROCKS. 



The minerals which are the principal constituents of rocks are the 

 following : — 



1. Those containing silica: as quartz; the feldspars; the micas; 

 hornblende ; pyroxene ; talc ; serpentine ; chlorite. 



2. Carbonates : as carbonate of lime, or calcite ; carbonate of lime 

 and magnesia, or dolomite. 



3. Sulphates : as sulphate of lime or gypsum. 



Th especial characteristics of these, and of other less frequent min- 

 eral constituents, will be learned from a Manual of Mineralogy. The 

 following are the prominent characters of the most common kinds : 1 — 



(1.) Quartz. — Quartz is the first in importance. It occurs in crys- 

 tals, like Figs. 32 and 33 ; also massive, with a glassy lustre. It is too 

 hard to be scratched with a knife. It varies in color from white or 



1 The ordinary characters by which minerals are distinguished are — relative hard- 

 ness, as ascertained by a file, or the point of a knife, or by scratching one mineral with 

 another; specific gravity, or relative weight; lustre and color ; crystalline, form ; cleavage 

 (cleavage being a facility of cleaving or breaking in some one or more directions, and 

 affording even, lustrous surfaces, as in mica, gypsum, feldspar); fusibility ; chemical 

 composition. 



