430 



THE GEOLOGIST. 



also have occun-ed, entirely altering the dissolving and precipitating 

 power of the circulating fluid, and consequently giving us, in the 

 newer fissures, a set of minerals, which although they existed in 

 circulation in the older fissures, could not be precipitated in con- 

 sequence of the higher temperature then existing keeping them in 

 solution ; while, similarly, the newer fissures could not contain many 

 of the minerals found in the older ones, inasmuch as the temperature 

 had now so fallen as to render the circulating fluid incapable of 

 holding them in solution. 



3. The analogies found to exist between the relative age and general 

 characteristics of veins in countries widely ajpart, which gave rise to 

 the theory of " formations," are accounted for by assuming them to 

 be the "everywhere tolerably analogous consequences of local 

 eruption, which may have been very far separated from each other 

 by time." The supposed synchronous "formations" merely "re- 

 presenting the same stadium of local activity," each stadium 

 being assumed to produce results tolerably analogous, however 

 widely removed in time and distance each isolated process may 

 have occurred. 



4. Cotta also finds in the the structure of the Freiberg veins a proof 

 of the general truth of his hypothesis. The older veins he finds to 

 be generally massive, while the newer have a banded, or layer -like 

 form ; these structures, in both cases, being such as might a ]jriori 

 have been expected. 



5. "With regard to the infiltrative origin of veins, it is admitted to 

 be possible, although not probable, for all ore-veins ; as to the newer 

 and banded veins such an origin seems to be accepted. Tin-ore is 

 considered as not being found in purely infiltrative veins, although 

 circumstances are referred to which seem to indicate that there are 

 cases where that ore cannot have otherwise originated. 



X. General remarJcs. — Although this hypothesis afibrds a general 

 solution of many obscurities, it is not pretended that it is itself with- 

 out difficulties : the whole question of metalliferous deposits is far 

 too involved with various compHcated geological phenomena to be so 

 lightly dealt with. The hypothesis is at best a suggestion for con- 

 sideration ; but it has this advantage, that it embodies in a definite 

 form a set of facts, -with which it harmonizes admii'ably. The 



