28 THE MINERAL CONSTITUENTS OF ROCKS. 



in the rock mass itself, or is carried out and deposited in any contiguous 

 cavity or fissure, or borne away to be deposited from thermal or mineral 

 springs, or from bog, river, lake, or ocean M^aters. 



The efforts to exjilain the changes in rocks and in their mineral con- 

 stituents by theories of pseudomorphism have generally fliiled, because the 

 changes have been attributed to the single minerals, and not to the rock 

 mass as a whole. 



The concentration of ores in rocks, and the formation of mineral veins 

 seem to be brought about by the same process as the more common altera- 

 tion of the rock mass, or the storing up of the material in minute fissures 

 in tbe rock.* The only difference is that the kind of material and its 

 amount, owing to the size of the receptacle and the extent of the action, 

 is such as to make it commercially valuable. In this statement there would 

 be excepted all ores that can be proved, as some iron ores have been, to be 

 of eruptive origin, as well as all mechanical deposits. In this connection it 

 may be explicitly stated, the writer holds the view that the elements of 

 most of tlie ores were disseminated through the original and eruptive rocks, 

 and that when these rocks became exposed to the action of meteoric 

 agencies, these scattered materials were collected and deposited in the veins 

 and segregations in which they are now formed. f So far as now known, 

 the only ore of eruptive origin, in masses sufficient for exploitation, is that 

 of iron, which is so only in part of its occurrences. If this view is cor- 

 rect, it would follow that our veins and most of our ore deposits are superfi- 

 cial phenomena t of the earth, and that mineralogy and economic geology 

 as ordinarily studied relate chiefly to the secondary products of mineral 

 matter; or, they are the sciences of abnormal minerals. 



The above described alterations in rocks and minerals, and the localiza- 

 tion of mineral deposits, with the consequent essential original unity of 

 ancient and modern rocks, naturally follow from the general law of the 

 passage from the unstable towards a more stable condition. This results 

 from the fact that the original materials of the earth, whether forming the 

 original crust, or appearing on the surface as eruptive rocks, are in a higher 

 state of energy than is adaptable to the surface conditions of this globe. 

 They are unstable both in temperature and in the majority of chemical 

 combinations formed on solidification, and heat is lost with a resulting 



* Bull. Mus. Comp. Zool, ISSO, vii. 123-130; Proc. Bost. Soc. Nat. Hist., 18S0, xxi. 91-103. 

 t Eug. Mill. Jour., New York, 1884, xxvii. 364, 365. 

 + Whitney, Aurif. Gravels, pp. 350-361. 



