218 SIXTEENTH REPORT ON THE CABINET OP NAT. HISTORY. 



of Lake Superior, may have marked the coast-line from which 

 the materials of the formation were derived, while the finer sand 

 was precipitated in the deeper ocean along the abruptly shelving 

 coast to the southward. 



In this sheltered position, accumulating to great thickness, 

 while the gradual depression of the ocean-bed was going on, it 

 has preserved, in all its stages, the phenomena of beach-lines, 

 ripple and wave-lines, and even mud-cracks, with fucoidal remains 

 in its lowest beds; aifording at the same time favorable habita- 

 tions for myriads of crustacean forms, and certain Mollusca which 

 mark its successive stages. At the period when these shallow 

 water or beach-lines are indicated in the sandstone of Lake Pepin 

 and Trempaleau, the conditions farther to the south may have 

 favored the accumulation of a magnesian limestone*. 



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 pe- 

 riodic 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 portions 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. Consequently the source 

 of this higher sandstone, still in part the coast-line of the Huro- 

 nian 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 unconforraably 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 ho- 



* 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 por- 

 tions of the formation, dolomitlc depositions have occurred. 



