20 I. C. RUSSELL CONCENTRATION AS A GEOLOGICAL PRINCIPLE 



The most common geological deposits which owe their concentration 

 to the process under consideration (not at present considering water as a 

 part of the category or attempting to strictly define sublimation as a 

 physical-chemical process) are sulphur, arsenic, mercury, etcetera. 

 Owing to the high temperatures present in volcanoes, inclusive of their 

 funnels and solfatara stages, and the variety of substances usually pres- 

 ent, the chemical activity is conspicuous, and it is difficult to differentiate 

 the concentrates resulting from sublimation simply from the results pro- 

 duced by chemical reactions, evaporation, etcetera. For the present at 

 least it seems best from a geological point of view to make a special class 

 of the concentrates resulting from the direct action of the earth's inter- 

 nal heat, when- transposed to the earth's surface, and term these fuma- 

 rolic and solfataric products. The concentrates of this class produced 

 essentially in the absence of water, except in the condition of steam, are 

 analogous to hot-spring deposits, in the production of which the earth's 

 internal heat is the dominant agency, in association with water. In fact, 

 the concentrates formed in connection with fumaroles and solfataras may 

 be considered as the products of hot-gas springs. In a manner similar 

 to the way in which the deposits froin hot-water springs grade into other 

 forms of aqueous deposition, so the fumarolic deposits merge with other 

 accumulations resulting from the action of volcanic heat at the surface 

 or in the rocks through which the conduits of volcanoes pass. 



Another phase of volcanic activity, which, however, can at present be 

 considered but briefly, is the concentration of gases arising from the dis- 

 association of the elements of substance exposed to the high temperatures 

 prevailing in volcanic vents. By this process various gases result, as, for 

 example, hydrogen, oxygen, nitrogen, ammonia, etcetera, and various 

 acids in a gaseous condition; but in general and perhaps always, so far 

 as the geologist is especially concerned, these concentrates, owing to their 

 great chemical activity, and also, as a nearly universal rule, the absence 

 of suitable reservoirs in which they can accumulate and be permanently 

 stored, do not demand attention as geological deposits. 



CHEMICAL REACTION RESULTING IN PRECIPITATION 



A highly varied group of natural chemical concentrates have as the 

 principal or controlling agency leading to their production what is 

 termed chemical reaction, or the mutual disassociation of the elements 

 in two or more compounds when brought into intimate relations with one 

 another under certain conditions of temperature, pressure, etcetera, and 

 the production of one or more new compounds which may separate out 

 and be placed by themselves. For example, if a water solution contain- 



