16 S. F. EMMONS — THEORIES OF ORE DEPOSITION 



already charged by direct emanations. It is difficult to account for the 

 gangue minerals as direct emanations, since they are not volatile except 

 such as are combined with fluorine. Certain deposits without gangue in 

 eruptive rocks and deposits in limestone in contact with eruptive rocks, 

 associated with garnet, ilvaite, and similar minerals, may have been 

 deposited by sublimation, but these are exceptional. For most veins he 

 admits, in accord with Bischof, that the earth)' - minerals must have come 

 from the decomposition of the country rocks. The greater proportion of 

 true veins (veins of incrustation) he considers to have been formed by 

 deposition from waters circulating in cracks in the earth's crust. In 

 this, his theory resembles Werner's, but differs from it in assuming that 

 the solutions were ascending rather than descending. Werner's argu- 

 ment in favor of descending waters, namely, that veins become poorer in 

 depth, he considers not well founded, the facts of nature rather going to 

 prove that the solutions became weaker as they approached the surface. 



Stanniferous veins, which contain a great number of the rarer ele- 

 ments and are associated with acid rocks, are the type of the first class, 

 while ordinary or plumbiferous veins, which are characterized by the 

 important role of mineralizers and the absence of anhydrous silicates, are 

 usually associated with basic rocks. 



His reasoning is evidently based largely on his observations in Corn- 

 wall, and on an assumed difference in the origin of granite and of vol- 

 canic rocks in general. Granite, he assumes, owes its crystallinity less to 

 the fact of having crystallized at great depths than to its content of water 

 (2 to 3 per cent), which enabled it to remain in a pasty condition much 

 below its fusion point, and thus allowed quartz to take the impress of 

 other minerals. The minerals of the first group of veins form part of 

 the outer crust (penumbra) of granite bodies, as granite once formed the 

 outer crust of the earth. 



The products of volcanic eruptions he divides into two classes : (1) the 

 lava-like, which consist of silicates in a molten condition; (2) the sul- 

 phur-like, which emanate in a molecular condition. To the latter alone 

 can the formation of vein minerals be attributed. The term " solfataric," 

 which was employed by subsequent writers to describe the action of the 

 sulphur-like emanations, has since been very generally used by writers 

 on ore deposits in a sense which is not always strictly accurate. 



Although de Beaumont's views are based on some premises no longer 

 considered tenable, they mark an important advance in this line of re- 

 search, in that they may be said to be the first fruits of organized field 

 investigation, for the first geological surveys, those of Great Britain and 

 of France, had been founded about ten years before, and the first geo- 

 logical map of France had recently (1841) been completed by the latter 

 survey, of which he was the founder and director. 



