46 GEOLOGY OF THE BLACK HILLS. 



they are found to sustain an unconforming relation to one another, but 

 elsewliere, where the Eozoic has been divided into these periods, or sup- 

 posed to be so divisible, the separation rests almost exclusively upon the 

 slender ground of lithological characters. It is true that to a certain extent 

 particular rocks or constituent minerals are found to be characteristic of a 

 particular formation; but such evidence alone cannot be taken as conclusive 

 in co-ordinating series of rocks, especially where the points of observation 

 are separated by any considerable distance. At the best, a similarity of 

 lithological structure is only presumptive evidence as to similarity of age. 



In rocks of the Laurentian the Canadian geologists discovered a pecu- 

 liar structure concerning the character and relations of which there has 

 been no little discussion. It is considered by Carpenter, Dawson, and 

 others to be a fossil, a gigantic foraminifer, and has been described b}^ its 

 first student, Dr. Dawson, as Eozoon Canadense. A similar structure has 

 since been recognized from the Eozoic of Bohemia and Scandinavia. This 

 is the oldest remnant of organic life yet discovered, and it is the main 

 foundation for the primary division of the Archaean into Azoic and Eozoic. 

 The presence, however, of large beds of limestone, graphite, apatite, and 

 iron ore in the Eozoic rocks is strong evidence of the existence of organic 

 life, for life appears in all observed modern processes to be necessary to the 

 separation or accumulation of such deposits. 



The Laurentian, where best known, is composed chiefly of granitoid 

 rocks, gneiss, quartzite and crystalline limestone. It is frequently charac- 

 terized by hornblendic rocks, but is seldom very micaceous; and the pres- 

 ence of graphite and large beds of magnetic and titaniferous iron ore in 

 many regions prominently characterizes the series. The system was first 

 studied by Sir William Logan in a field where it is largely developed — the 

 Canadian highlands bordering the Saint Lawrence and the Great Lakes and 

 extending far northwestward. The Archaean rocks of the Adirondacks, at 

 least a portion of those in the Alleghanies, those of the Ozarks in Missouri, 

 and a portion if not the larger part of the iron-bearing rocks of the Lake 

 Superior region, are considered to be of Laurentian age, and in all these 

 regions the system is characterized by immense beds of iron ore. 



In Scotland the "fundamental gneiss" has been regarded by Murchison 



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