Contributions to Palceontology. 



189 



Trempaleau, the conditions farther to the south may have 

 favored the accumulation of a magnesian limestone. 1 



The force of the oceanic current was apparently not 

 sufficient to transport to great distances the arenaceous 

 sediment ; and to this cause is probably due its attenuation 

 in Missouri, and its almost entire absence farther to the 

 south. 



The depression of the coast-line, whether a constant or 

 a periodic movement, would be attended by a more or less 

 marked change in the character of the sediments. At 

 the time of the accumulation of the St. Peters or Chazy 

 sandstone, not only was the Potsdam sandstone of the 

 Upper Mississippi valley far beneath the sea-level, but, 

 inferring from the undisturbed condition of adjacent por- 

 tions of the country, the source of the materials of the 

 older sandstone, the Huronian coast-line, had likewise 

 become depressed beneath the level of the ocean. Conse- 

 quently the source of this higher sandstone, still in part 

 the coast-line of the Huronian rocks was removed farther 

 to the north, and probably may have included to some 

 extent the Laurentian formation. 



Admitting this view of the case, we should expect to 

 find the older conglomerates of Lake Superior overlaid 

 unconformably by the sandstone of the newer period; 

 while the conglomerates of the latter would be found still 

 farther to the north, marking the outline of the ancient 

 coast. We are met, however, with serious objections to 

 this view ; for the conglomerates of the north and east 

 shores of Lake Superior are uptilted and cut by numerous 

 trap dykes, while the sandstone of the south shore is nearly 

 horizontal, and not penetrated by trap dykes, which some- 



1 There are at intervals in this sandstone, from near its base, thin bands 

 which have an apparent magnesian character; so that even in the well- 

 marked arenaceous portions of the formation, dolomitic despositions have 

 occurred. 



