PRESENT PROBLEMS OF GEOPHYSICS 515 



these, or even an equivalent mechanical stirring, would necessarily 

 lead to fractional crystallization, a familiar process known even to 

 the pupils of Aristotle, and which is almost unavoidable when mixed 

 solutions solidify. This process is one of precipitation, and is abso- 

 lutely distinct from the differentiation (or, more properly, segrega- 

 tion) of rock magmas, in which a single liquid is supposed to separate 

 into two or more distinct liquids. The general conditions of the order 

 of precipitation during fractional crystallization in accordance with 

 the phase rule are by no means beyond the reach of discussion, and 

 the able investigations of Messrs. J. H. L. Vogt and J. Morozewizc 

 have a direct bearing on this subject. 



A mystery which will assume greater importance as the accessible 

 supply of coal diminishes is the origin of petroleum. There is much to 

 be said in favor of the unpopular hypothesis of Mendeleef , supported 

 by experiments on cast-iron, that liquid hydrocarbons are due to the 

 decomposition of the iron carbides of the terrestrial nucleus. Such 

 vast accumulations of oil as exist on the Caspian and in the Caucasus 

 seem incompatible with the hypothesis of animal or vegetable origin, 

 although oils belonging to the same series as do the petroleums have 

 been produced in the laboratory from organic materials. On the other 

 hand, some meteorites contain hydrocarbons (which may themselves 

 be due to the alteration of iron carbides), and there are geologists who 

 infer that the petroleum may be derived from the mass of the earth 

 itself. 1 If the origin of the oil is not animal or vegetable, the supply is 

 very likely inexhaustible. More extended study of the connection 

 between volcanic phenomena and the origin of asphaltic and other 

 hydrocarbons is a desideratum. 



Ore-deposits themselves form the branch of geology which was ear- 

 liest cultivated and which will never lose its interest so long as man- 

 kind remains gainful. Yet much remains to be done by experiment 

 for the theory and practice of mining-geology. The mechanism of the 

 secondary enrichment of ores, particularly those of copper, detected 

 by Mr. S. F. Emmons and enlarged upon by Mr. W. H. Weed, is be- 

 ing studied experimentally in the laboratories of the U. S. Geological 

 Survey. A feature deserving careful experimental study is the osmotic 

 separation of ores from their solutions by the wall-rock. Many minu- 

 tiae of occurrence suggested that the walls of veins often act as a species 

 of diaphragm or molecular filter and have a dialytic action on the ore 

 solutions. 2 The origin of the ores themselves is still very obscure and 

 will hardly be elucidated until more is known of the earth's interior. 

 Sometimes they seern to be derived from adjacent rocks; in other 

 cases conditions suggest that the rocks and the veins derive their 



1 See H. L. R. Fairchild, Bulletin of the Geological Society of America, vol. xv, 

 1904, p. 253. 



2 Mineral Resources of the U. S. for 1892, p. 156. 



