October 28, 1904.] 



SCIENCE. 



551 



guineous rocks. If the separation is not 

 molecular, it is self-evident that it must be 

 molar. The only molar currents readily 

 conceivable in a body of magma are con- 

 vection currents, and these, or even an 

 equivalent mechanical stirring, would nec- 

 essarily lead to fractional crystallization, a 

 familiar process known even to the pupils 

 of Aristotle, and which is almost unavoid- 

 able when mixed solutions solidify. This 

 process is one of precipitation and is abso- 

 lutely distinct from the differentiation (or 

 more properly, segregation) of rock mag- 

 mas, 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 crystalliza- 

 tion in accordance with the phase rule are 

 by no means beyond the reach of discus- 

 sion, and the able investigations of Messrs. 

 J. H. L. Vogt and J. Morozewicz have a 

 direct bearing on this subject. 



A mystery which will assume greater im- 

 portance 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 ex- 

 periments on cast iron, that liquid hydro- 

 carbons 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 or- 

 ganic 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.* If the 

 origin of the oil is not animal or vegetable, 

 the supply is very likely inexhaustible. 



* See H. L. R. Fairehild, Bull. Geol. Soc. Amer., 

 Vol. 15, 1904, p. 253. 



More extended study of the connection be- 

 tween volcanic phenomena and the origin 

 of asphaltie and other hydrocarbons is a 

 desideratum. 



Ore deposits themselves form the branch 

 of geology which was earliest cultivated 

 and which will never lose its interest so 

 long as mankind remains gainful. Yet 

 much remains to be done by experiment 

 for the theory and practice of mining geol- 

 ogy. The mechanism of the secondary en- 

 richment of ores, particularly those of cop- 

 per, detected by Mr. S. F. Emmons and 

 enlarged upon by Mr. W, H. Weed, is be- 

 ing studied experimentally in the labora- 

 tories 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 

 minutife 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.* 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 seem to be derived from 

 adjacent rocks ; in other cases conditions 

 suggest that the rocks and the veins derive 

 their metallic content from a common deep- 

 seated source. Here, as in several other 

 connections. Professor Suess's theory of 

 'juvenile waters' is very suggestive. It is 

 held that many of the great iron deposits 

 are due to magmatie separation. Deposi- 

 tion of lead ores by replacement of calcite 

 is a known process, but takes place under 

 unknown conditions. In some cases re- 

 placement of rock by ores appears to me 

 to be alleged without sufficient proof. 

 Pseudomorphosis is the only adequate test 

 of replacement. 



Erosion appears to be a subject which 

 is capable of more exact treatment than it 

 has received. Weathering and abrasion 



*'Min. Resources of the U. S. for 1892,' p. 1.56. 



