Geology of the Great Lakes and the Mississippi. 227 



We have seen in the preceding pages that the term " Greensand " 

 was originally applied by Smith and Wobstor to tlio bod now known 

 as Upper G'reensand, and that, througli the excusablo, but most unfor- 

 tunate, misconceptions of succeeding writers, it came to bo applied 

 to a formation of much older date ; also how, in an ineffectual 

 attempt to get rid of the confusion which had arisen, the words 

 Upper and Lower came to be added to the terra. We have also seen 

 the great objections there are to the term " Lower Grecnsand," and 

 the attempts which have been made in this country to find a substi- 

 tute for it. Lastly, wo have followed the labours of those geologists 

 in Switzerland and France, who, studying the same formation in its 

 more complete development, have suggested a name in every respect 

 suitable — one against which no valid objection has ever been urged, 

 and which has taken a firm root in geological literature. 



It is for these reasons that we believe that the term Neocomian 

 has claims before all others to be applied to the great system of 

 strata between the Cretaceous and the Oolites. Terms like Silurian 

 and Neocomian are the monuments of great epochs in the history of 

 geological discovery, and the former is not more closely associated 

 with the achievements of Murehison than is the latter with the 

 labours of de Montmollin. 



^roTxczBS OIF ^v^snycoiiis-. 



I. — On the StrRFACE-GEOLOGT OF THE BaSIN OF THE GrEAT LaKES, 



ANDTHE Valley of thbMississippi. . By Ptof. J. S.- Newberet, M.D. 

 [Annals of the Lyceum of Nat. Hist., New York, Vol. IX., 1869.] 



THE area under consideration is bounded on the north by the 

 Eozoic highlands of Canada, on the east by the Adirondacks 

 and Alleghanies, and on the west by the Rocky Mountains. 



Marks of glacial action are abundant in the northern half of the 

 area, and where the nature of the rocks is such as to retain in- 

 scriptions, there numerous furrows are found, having a general north 

 and south direction. 



In the southern half of the Mississippi valley the evidences of 

 glacial action are entirely wanting, and there is nothing corresponding 

 to the wide-spread Drift deposits of the north. The points of chief 

 interest are the proofs which it furnishes- of erosion on- a stupendous 

 scale. 



Some- of the valleys and channels (in the northern' area) which 

 bear the marks of glacial action— evidently formed or modified by 

 ice, and dating from the Glacial period or an earlier epoch — are 

 excavated far below the present lakes and water-courses which 

 occupy them. These valleys form a connected- system- of drainage, 

 at a lower level than the present river system. 



Lakes Michigan, Huron> Erie, and Ontario, are basins excavated 

 in undisturbed sedimentary rocks. An old excavated, but now filled 

 up channel connects Lakes Erie and Huron. Another trough runs 



