St. Peter's Sandstone. 



12- 



In the twelve feet of sand and shale given in the above 

 section, we have transition beds between the true Trenton 

 limestone and the typical St. Peter's sandstone below. It 

 would appear that after the deposition of the white sandstone 

 there came a period of muddy seas, during which the green- 

 ish shales were laid down. Then followed a period of eleva- 

 tion to near the surface, when the loose sand and the harder 

 beds below were deposited. The latter were probably 

 cemented together by lime from the layers of limestone 

 above, which were in their turn laid down in a deepening 

 sea, that soon swarmed with life. 



In a late paper by Messrs. Hall and Sardesoir ;: a similar 

 conclusion is reached. These authors say that they nowhere 

 found in Minnesota any indication of an uncomformity be- 

 tween the Trenton and St. Peter's. "The transition zone of 

 a green shaly calcareous sandstone shows the steady oncom- 

 ing of that Lower Silurian sea which, if it did not submerge 

 the whole Northwest, at least extended so far that the dry 

 land was reduced to islands, or narrow peninsular stretches of 

 very uncertain connection with a mainland lying somewhere." 



STRATIGRAPHIC POSITION. 



The position of the formation in the stratigraphical col- 

 umn is unequivocal. It lies between the Lower Magnesian 

 limestone and the Trenton limestone, but its limited area in 

 the country, and its practical isolation from eastern localities, 

 renders its correlation with formations elsewhere a matter of 

 difficulty. In addition to this the almost total absence of 

 fossils adds to the perplexity, as it is by means of the organic 

 forms that rocks are most accurately correlated. The only 

 point where direct stratigraphic continuity with any eastern 

 North American formation can be looked for is on the Cana- 

 dian side of the Sault Ste. Marie. Its presence here is shown 

 on the geological map of Canada by Hall and Logan, t though 

 it is present to only a limited extent. It does not occur 

 again, in outcrop at least, until the Ottawa River is reached. 



-Bull. Geol. Soc. America, Vol. III. p. 355. 



t Atlas accompanying Geology of Canada from commencement to 1S63. 1865. 



