

LOWER SILURIAN. 



181 



In the next or Potsdam epoch, there were beach deposits of sand in 

 progress about the shores of the Archaean dry land, but in Vermont 

 mostly shales, with some limestones, indicating deeper waters off the 

 Archaean coasts. West of Vermont, this coast line bent around the 

 Adirondack region of northern New York and Canada, as marked 

 out by the distribution of the Potsdam. The Potsdam rocks of New 

 York and Canada indicate their beach or shallow-water origin, by their 

 foot-prints, worm-borings, ripple-marks and mud-cracks (p. 169). Simi- 

 lar evidences of shallow water are observed also in the Potsdam rocks 

 of Pennsylvania and Tennessee. Thus we are enabled to run a line 

 of soundings along the continental sea of the Potsdam era. The ma- 

 terials of the sandstones were the moving sands and pebbles of the 

 shores and shallow seas ; and the animals which had living places over 

 tnese flats and sea bottoms found in them also a burial place, to remain 

 as fossils and become testimony as to the early life of the world. The 

 accompanying sketch, by J. D. Whitney, represents the Archaean at 

 Carp River, Michigan, overlaid unconformably b^ beds of Potsdam 



Fig. 283. 



Unconformability at Carp River, Michigan. 



sandstone — a portion of the " Pictured rocks " of the southern shores 

 of Lake Superior. They are here shown to be the sand-beds that 

 were made in the seas of the era, against a seashore ledge of crystal- 

 line Archaean rocks ; and that finally became consolidated, and also 

 somewhat tilted. 



2. Climate. — No marked difference between the life of the Pri- 

 mordial period in warm and cold latitudes has been observed ; and 



