W. J. McGee — Geology of the Mississippi Valley. 417 



Interpretation of the Record. 



Without the least disposition to discuss mooted points, especially 

 as regards the relative efficiency of land-ice and floating bergs in 

 laying down finely comminuted and coarser materials without 

 arrangement, the writer may briefly summarize the conclusions 

 to which he has been led by the study of the surface deposits of 

 the regions described. 



Member number six is believed to be the product of a continental 

 glacier because (1) it contains pebbles of the Arckeean rocks, which 

 must have been brought from a distance of from 200 to 400 miles, 

 mixed with a larger proportion of local ones ; and because (2) it 

 occurs at various levels, and is unstratified. It is believed to be 

 distinct from, and much older than the incumbent members, because 

 its materials have undergone much more secular modification ; and 

 by far the greater part of this took place before the advent of the 

 succeeding glacier. The cementing of a part of its materials into 

 pudding-stone occurred previous to any subsequent cataclysm, as is 

 attested by the finding of this pudding-stone in all of the more 

 recent formations. Even its erratic lumps of clay are easily dis- 

 tinguishable from their surrounding materials, and their true 

 relations readily traced. 



Members number four and five also contain northern rocks, often 

 striated, and occur at all levels; and their materials are often con- 

 fusedly intermingled. Lumps of the blue clay occur also in member 

 number three ; and in Noi'th-eastern Iowa the boulders of these 

 members are of a different character from those of the next. They 

 are therefore considered to be of glacial origin also, and to be 

 distinct from and much older than the overlying deposits. In 

 addition to the evidence above detailed, we have every reason to 

 believe that the accumulation of a great mass of vegetation, and the 

 development of a characteristic fauna, 1 took place subsequently to 

 the laying down of these members and previous to the coming of 

 the latest glacier. Only during an immense period could this have 

 been accomplished. 



For reasons which every geologist will recognize in reading its 

 description (even if he does not admit their adequacy), member 

 number three is believed to be of glacial origin. It is so considered, 

 indeed, by most of the various State and other geologists who have 

 examined and described it. The reasons for considering it to be 

 distinct from and much newer than the lower till, need not be 

 rehearsed. 



Partly in consequence of a high estimation of the opinions of 

 many eminent men, member number one is provisionally assumed 

 to be due to sedimentation in local basins formed at or about the 

 close of the last glacial epoch ; and member number two is thought 

 to be the coarser materials left when the lighter portion was carried 

 away to form the loss-like clay. As previously intimated, there is 

 reason for believing that a part, at least, of this assortment took 



1 Some references are given in the paper already mentioned, which bear upon 

 this point. 



DECADE II. — VOL. VI. — >*0. IX. 27 



