54 LITHOLOGICAL GEOLOGY. 



equally common pyroxene, characteristic of the heavy, dark-colored 

 lavas. 



(9.) Carbon. — Carbon is well known in three different states, — that 

 of the diamond, the hardest of known substances, that of graphite 

 or black lead, and that of charcoal. Combined with oxygen it 

 forms carbonic acid (CO 2 ) ; and carbonic acid combined with lime 

 makes carbonate of lime, or common limestone ; with magnesia, 

 carbonate of magnesia, or magnesite ; with protoxyd of iron, car- 

 bonate of iron, or spathic iron ; etc. 



Carbonic acid exists in the atmosphere, constituting ordinarily 

 about one part in twenty-five hundred by weight. 



This acid is the only acid in the mineral kingdom, in addition to silica, which 

 enters very largely into the constitution of rocks; and, while silica has alumina 

 and other sesquioxyds wholly to itself, carbonic acid shares with it in the mag- 

 nesia, lime, and alkalies, that is, in all the protoxyds. Carbon, we have said, 

 performs as fundamental a part in living nature as silicon in dead nature ; and 

 it is mainly through living beings that it reaches the mineral kingdom and forms 

 limestones and coal-beds. The deposits 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, aluminium, 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 combination with various minerals it forms ores called sulphurets, as sul- 

 phuret of iron, or pyrites, sulphuret of copper, sulphuret of silver. But these 

 sulphurets do not constitute properly beds of rock; although one of them, 

 pyrites, is very abundant. Sulphur forms with oxygen two acids, sulphurous 

 acid (SO 2 ), and sulphuric acid (SO 3 ). Sulphuric acid united with lime makes 

 sulphate of lime, or gypsum, which sometimes occurs in extensive beds. There 

 are also many other sulphates, but none as 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, as well as 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 3 ) ; 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 

 connected with chemical geology, in which department the pro- 

 foundest knowledge of chemistry and mineralogy is none too much. 

 But in a general review of rocks only these thirteen need be con- 

 sidered. 



