4 KEMPS ORE DEPOSITS. 



generally adopted. For clearness in illustration, the equivalent 

 terms employed by Dana are appended. 



The United States Geological Survey divides as follows : Era 

 and System, Period and Group, Epoch and Formation. In consider- 

 ing the ore deposits of the country, we employ only the physical 

 terms. We understand, of course, the chronological position 

 of the systems in the historical sequence, but it is of small moment 

 in this connection what may be the forms of life inclosed in them. 

 The purely physical character of the rocks whether crystalline or 

 fragmental ; whether limestone, sandstone, granite, or schists ; 

 whether folded, faulted, or undisturbed are the features on which 

 we lay especial stress. In all the periods the same sedimentary 

 rocks are repeated, and in the hand specimen it is often impossible 

 to distinguish those of different ages from one another. The 

 classification, briefly summarized, is as follows : 



1.01.02. ARCHAEAN GROUP. I. Laurentian System. II. Hu- 

 ronian System. Additional subdivisions have been introduced by 

 Canadian and Minnesota^geologists (Animikie, Montalban, etc.), 

 and there is a tendency to group all the later schistose members, 

 especially in the region of the Great Lakes, under the name Algon- 

 kian. (See discussion under Example 9.) 



PALEOZOIC GROUP. III. Keweenawan System. (This may be- 

 long with the Archaean.) IV. Cambrian System : (a) Georgian 

 Stage ; (b) Acadian Stage ; (c) Potsdam Stage. V. Lower Silurian 

 System. (A) Canadian Series : (a) Calciferous Stage ; (b) Chazy 

 Stage. (This will probably experience revision.) (J?) Trenton 

 Series : (a) Trenton Stage ; (b) Utica Stage ; (c) Cincinnati or 

 Hudson River Stage. VI. Upper Silurian System. ( A) Niagara 

 Series : (a) Medina Stage ; (b) Clinton Stage ; (c) Niagara Stage. 

 (J3) Salina Series. ( C ) Lower Helderberg Series. (D) Oriskany 

 Series. (The Oriskany may be made the base of the Devonian.) 

 VII. Devonian System. (^L) Corniferous Series : (a) Cauda-Galli 

 Stage ; (b) Schoharie Stage ; (c) Corniferous Stage. (B) Hamilton 

 Series : (a) Marcellus Stage ; (b) Hamilton Stage ; (c) Genesee 



