62 STRUCTURAL GEOLOGY. 



carbon, carbon dioxide (or carbonic acid), CO.; with (10) sUlcon (the name 

 from the Latin silex, flint), silica, SiOg. 



These and other essentially stable oxides are the chief constituents of rock- 

 making materials. They are in strong contrast with the compounds that 

 make up organic tissues, or those of plants and minerals. These coutain, 

 along with the oxygen present, carbon, hydrogen, nitrogen, and generally a 

 little sulphur and phosphorus, elements that have a strong afiinity for 

 oxygen, but they are associated with too little oxygen to satisfy their affiui- 

 ties, and, moreover, all are under a degree of restraint from the living 

 conditions. When these conditions are removed at death, ordinary chemical 

 affinities rule, and oxides are formed out of the elements of the tissues, and 

 of outside as well as inside oxygen — COo, CO, H2O being the chief products. 

 If outside oxygen is mainly excluded during the decomposition, hydrocar- 

 bon compounds form, or those that constitute mineral coal, oil, gas, and the 

 black or brown carbonaceous material that colors soil and many rocks ; but 

 these on burning become mostly CO2 and HgO. 



Carbon. — Carbon is a prime element in living structures, as silicon is in 

 rock-making minerals. In its pure state, crystallized in octahedrons and 

 related forms, it is the diamond, the hardest of minerals. Crystallized in 

 six-sided tables or scales of a dark lead-gray color, it is graphite (or plum- 

 bago), one of the softest of minerals ; often called " black lead," because it 

 leaves a trace on paper much like, but darker than, that of lead. Substances 

 having like composition, but different in crystallization, as diamond and 

 graphite, are called paramorphs. Charcoal is nearly pure carbon, but 

 contains some hydrogen and oxygen; and the best mineral coal is only 

 75 to 85 per cent carbon. Carbon combined with oxygen, forming CO2, or 

 carbon dioxide, is given out in the respiration of animals, and is thus 

 contributed to the air, and by aquatic animals to the waters, and is a large 

 result, as before explained, of all decay. At the same time, it is the source 

 of carbon to the growing plant. Carbon dioxide has great geological 

 importance through its combination with lime (CaO), producing calcium 

 carbonate, the formula of which is CaCOs (or its equivalent CaO -f CO2), 

 the material of ordinary limestone. 



Silicon. — Silicon combined with oxygen, and thus making silica (SiOj), 

 constitutes the two minerals, quartz and opal. Quartz is the most abundant, 

 durable, and indestructible of common minerals. Silica also enters into 

 combination with various oxides, and thus makes silicates. 



Of the oxides in these silicates, alumina, AI2O3, is the hardest, most 

 infusible, and most indestructible. Like silica, it is well fitted for a chief 

 place in the earth's foundations ; and next to silica it is the most abundant. 



Silica combined with alumina alone, makes only infusible silicates ; but if 

 potash, soda, lime, magnesia, or the oxides of iron are present, the minerals 

 in general are fusible, and are, therefore, suited for the material of a melted 

 as well as of a solid globe. 



