196 PALEOZOIC TIME — LOWER SILURIAN. 



limestone or shale, and the reverse. These oscillations are the 

 subject of brief remark in the next section, on the geography of 

 the period. 



IV. General Observations. 



1. North American Geography. — On p. 136 a map is given pur- 

 porting to represent the general outline of North America at the 

 close of the Azoic period. It is there stated that there may have 

 been other lands above the water, especially about the summits of 

 the Eocky Mountains and the regions beyond, making islands, 

 large and small, in the great continental sea ; but that the conti- 

 nent, in a general way already defined as to its ultimate outline, 

 lay at no great depth beneath the surface of the water. The facts 

 gathered from the rocks of the Primordial period throw additional 

 light on early American geography. 



(1.) Northern border of the interior basin. — We learn from the beds 

 of the first or Potsdam epoch that along the northern border of 

 the United States the waters were shallow, and that there were 

 beyond doubt coasts and exposed sand and mud flats. The ripple- 

 marks, so common in the strata both of New York, Canada, and the 

 West, must have been formed either along a sandy beach or over 

 a shallow bottom. The alternation of oblique and horizontal 

 lamination in many layers (fig. 61 e, p. 93) is evidence of ebbing 

 and flowing tides. The wave-lines show where the waves actually 

 dashed over the sands of a coast. The various inclinations of 

 the lamination (like fig. 61/, p. 93, drawn from the Michigan Pots- 

 dam beds) point out the rocks as once wind-drifts of sand that had 

 been decapitated again and again by storms ; and the tracks of 

 crustaceans, as Logan has observed, as well as the mud-cracks, 

 could only have been made upon land above the level of the sea. 

 The worm-burrows (Scolithi) in the sand-rocks were made in the 

 sands near tide-level, or not far below. In fact, sandstone itself, as 

 will be shown in a future chapter, is evidence to the same purport ; 

 for the sands as well as pebbles of marine formations are accumu- 

 lated only along shores and within a narrow range of soundings. 

 Hence the prevailing sandstones and conglomerates of the Pots- 

 dam formation indicate that there were no deep seas where these 

 rocks were laid down. Thus, from New York through Canada, 

 Michigan, Wisconsin, Minnesota, Iowa, and Nebraska, to the Black 

 Hills, and the Laramie Eange on the Eocky Mountains, we are en- 

 abled to run a line of soundings for the Potsdam period. When the 

 layers of the Potsdam sandstone that now skirt the Black Hills of 

 Dakota were forming, the hills stood above the sea, and the 



