POTSDAM PERIOD. 199 



probable that this change consisted in a deepening of the waters 

 through subsidence. In the Potsdam epoch the continent, when 

 the rocks were forming, lay for the most part near the water's 

 level, washed by the ocean's tides, — a condition favorable for the 

 accumulation of sand deposits. In such a case the waters would 

 be too impure from disturbed sediment for the formation of lime- 

 stone. Such an amount of subsidence was therefore required as 

 would give clear and pure waters, — perhaps not more than a hun- 

 dred feet ; for the limestone of the coral islands is all made within 

 that depth. Moreover, the alternations of sandy strata with lime- 

 stone, so marked in Missouri, imply several oscillations of level 

 over the interior basin during the Calciferous epoch. These oscil- 

 lations are also indicated by the succession of strata in the Quebec 

 group and in other parts of the Appalachian region. To the east, 

 at the Mingan Islands, there were sandstone deposits in the earlier 

 part of the Calciferous epoch, but others of limestone before its 

 close. 



(6.) Lake Superior Sandstone beds and Trap rocks. — The deposits of 

 the Potsdam period in the vicinity of Lake Superior differ from 

 others of the interior basin in their great thickness (p. 195) ; but they 

 are also peculiar in their connection with the eruption of igneous 

 rocks. The evidences of igneous eruption are very numerous on 

 both the north and south shores of the lake ; and Isle Royal, stand- 

 ing in the lake, abounds in trap rocks of this period.. Such a 

 region of fires might naturally be one of extensive subsidence, — no 

 uncommon phenomenon in volcanic countries. This would give 

 an opportunity for the formation of thick deposits ; while if the 

 waters had been permanently shallow the marine formations must 

 have been thin, as they are in the peninsula of Michigan ; for 

 they could not have much exceeded the depth of the waters. This 

 region of Lake Superior and the other great lakes lies directly 

 against the Azoic ; that is, it is between the region of progress on the 

 south, which was undergoing frequent changes of level through the 

 Silurian ages, and the more stable Azoic of the north. Here the 

 series of depressions were formed which are now the lakes ; and 

 these igneous eruptions through the fractured crust of the earth 

 seem to have been an incident in the subsidence that was producing 

 the basin for the largest of all the lakes, Lake Superior. Lake 

 Champlain also lies along the borders of the Azoic, and has the 

 Silurian of New England on its opposite side. 



The thick sandstones, conglomerates, and shales of the Huronian 

 series occur in this same lake-region, and seem to show that the 

 first commencement of the lake-history dates as far back at least 



