188 



Contributions to Palaeontology. 



represent two of the epochs noticed in the northern localities, 

 as indicated by the fossils of the Upper Mississippi valley. 



Regarding, for the present, the formations below the St. 

 Peters or Saccharoidal sandstone in Missouri as equivalent 

 in age with those of Wisconsin, we are compelled to recog- 

 nize the Third and Fourth Magnesian limestones of the 

 Missouri Reports (as well as the. Second and Third sand- 

 stones) as represented in the sequence by the sandstone of 

 the Upper Mississippi river. 1 



The material of this older sandstone has doubtless been 

 derived from the crystalline quartzose rocks of the Huro- 

 nian period, and which are still exposed in extensive 

 masses on the southern side of Lake Superior, rising from 

 beneath that formation. 



Taking this view of its origin we are, in the region of 

 the Upper Mississippi, not far from its source ; and the 

 older conglomerates at the base of the sandstone in the 

 central part of Wisconsin, which are similar to some of 

 those on the south shore of Lake Superior, may have 

 marked the coast-line from which the materials of the 

 formation were derived, while the finer sand was precipi- 

 tated in the deeper ocean along the abruptly shelving 

 coast to the southward. 



In this sheltered position, accumulating to great thick- 

 ness, 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 fucoiclal remains in its lowest beds; affording at the 

 same time favorable habitations 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 



1 The fossils described from the Third Magnesian limestone by Dr. Shu- 

 mard, do not, I believe, include any Trilobites ; but all the species bear 

 more analogy with those known in the Calciferous sandstone of New York 

 and of Canada, and Dr. Shumard regards this rock as of the age of the 

 lower magnesian limestone of Iowa, Wisconsin and Minnesota. 



