COMPARATIVE VALUE OF DIFFERENT COALS. 213 
lup slowly toward the surface through fissures, large and small, would have their 
solvent power diminished both by cooling and by relief of pressure, and must of 
necessity deposit in their courses and form mineral viens. But the solvent power 
of subterraneous waters is still further very greatly increased for most vein matters 
by the pressure of alkali in the form of alkaline carbonates or alkaline sulphides, 
or both. This is especially true of the commonest of vein stuffs, viz : quartz and 
lime carbonate, and the commonest forms of metaUic ore, viz.: metallic suphides. 
The solubiHty of silica in alkaline carbonated waters is well known, and with 
excess of carbonic acid in the waters all the earthy and metallic carbonates are 
also soluble. The solubility of many and probably of all metaUic sulphides in 
alkaline sulphides, especially with excess of hydrogen sulphide under pressure and 
super-heat, can no longer be doubted ; for iron sulphide and mercuric sulphide 
are now being deposited from such waters, both at Sulphur Bank and at Steam- 
boat Springs. 
Mr. .Christy and others have proved the solubility of mercuric sulphide under 
pressure and super-heat by actual experiment ; and these are among the most 
insoluble of metallic sulphides. It is certain, then, that metallic sulphides are 
soluble to a limited extent in alkaline sulphides, forming doubtless double sul- 
phides. It is certain, also, that the solubility is increased by super-heat and 
pressure. It is therefore also certain that hot waters containing alkaline carbon- 
ates and alkaline sulphides, circulating at great depth and therefore under heavy 
pressure, would take up silica, earthy and metallic carbonates and metallic sul- 
phides, and that coming up slowly toward the surface they would deposit these 
•substances in their courses, partly by cooling and partly by relief of pressure, 
and thus form metalliferous veins. Cooling and relief of pressure are the most 
useful causes of deposit, but not the only ones. Organic matters are of almost 
•universal occurrence in subterranean waters, and their agency in reducing metal- 
lic oxides and metallic salts is well known. Organic matter is a universal reduc- 
ing agent. 
The acids of organic decomposition may prove a reducing agent. Such in 
brief is an outline of a true theory of the genesis of metalliferous viens — a theory 
apparently confirmed by the study of causes now in operation at Sulphur Bank 
and Steamboat Springs, and probably many other places in California and 
Nevada. 
COMPARATIVE VALUE OF DIFFERENT COALS. 
General M, C. Meigs has lately issued a pamphlet in which he shows the 
results of experiments made under his direction to determine the caloric value of 
different varieties of coal. This has in it so much that is valuable that we con- 
dense from it what we consider very interesting data. The experiments include 
thirty-one different kinds of coal. Nearly all the varieties were American, some 
English and Scotch. These experiments were carried out to determine the rela- 
VII— 14 
