544 THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



the oxide of this metal is very frequeutly found, and sometimes in 

 large quantity, in and near the outcrop of such veins. Water takes 

 carbonic acid from the air, and, percolating downwards, converts a great 

 variety of mineral substances more or less completely into carbonates. 

 This is a process which may be said to be going on everywhere on a 

 grand scale. 



Calcareous spar (calcite) is one of the most coyimon of veinstones, 

 occurring in very large masses, both in regular veins and in those 

 masses which belong to the segregated form of metalliferous and min- 

 eral deposits. 



Again, carbonate of lime, or of lime and magnesia, is among the most 

 commonly occurring products of the alteration of eruptive rocks. 



Much of the crystalline limestone, or marble, which occurs in the 

 Azoic series, seems to us to belong to the segregated form of occur- 

 rence. Some of it is certainly of this character, as has been already 

 mentioned in connection with the question of the organic character of 

 the Eozoon. If any of it is really bedded limestone — that is, rock which 

 was formed contemporaneously with the formation of the beds with 

 which it is associated — it seems to us clear that there are stronger rea- 

 sons for believing that this has been the result of a chemical precipita- 

 tion, than that the calcareous material has been formed through the 

 agency of life. 



In default of other evidence of the presence of the results of organized 

 existence in the azoic rocks, it has been maintained by some that the 

 occurrence of ores of iron in extraordinary quantity in that series fur- 

 nished the desired proof. The facts are, however, that some at least of 

 the iron thus occurring is of eruptive origin ; that the oxide of iron is a 

 mineral commonly and abundantly found making an essential compo- 

 nent of volcanic rocks ; that metallic iron is so found in large quantity, 

 — in one region, at least ; that there is strong reason for believing that 

 metallic iron forms, if not the whole, at least a large part, of the earth's 

 interior ; and, finally, that a large portion of the material which comes 

 to us from outside our planet is metallic iron. All this, we think, is 

 amply sufficient for a refutation of the theory, that the presence of the 

 ores of iron is a proof of the existence of life at the time when the rocks 

 were formed in which those ores occur. 



That it should be seriously maintained that the presence of sulphur 

 is evidence of the presence of organic agencies, seems to us still more 

 extraordinary than that the ores of iron should be so regarded. Sulphur 

 is one of the most abundantly diffused of the elements. The sulphu- 



