292 



now living in our waters. This bank has evidently been formed 

 by the gradual wearing down of the bluff where it formed the 

 shore of the Lake, the material being spread out under water 

 like the delta formed off the mouth of a large river in the ocean. 

 The direction of the layers, parallel to the surface, is a signifi- 

 cant fact. This delta rises gradually from the marsh to the foot 

 of the bluff, where it attains an elevation of twenty-four feet 

 above the present water level. It may then be considered as an 

 established fact, that Lake Michigan, at no very remote period, 

 (certainly far subsequent to the drift period,) stood at an eleva- 

 tion of at least twenty-four feet above its present surface. The 

 difference in character of the light-colored materials of the delta 

 and the very hard blue and red clay is quite obvious, and could 

 not be confounded. There are other places where similar deltas 

 are formed, but they have not been opened so as to show their 

 geological character. I have no doubt the locality examined by 

 you at Chicago belongs to the same era ; for the banks there, 

 being only from six to ten feet high, must have been entirely 

 submerged." 



Dr. C. T. Jackson remarked, that he had pointed out, in 

 1844-5, that Lake Superior was formerly at least twenty feet 

 higher than the present level. 



The following shells from the United States Exploring 

 Expedition were described by Dr. Gould : — 



Cyclas egregia. T. ventricosa, transversa, oblonga, sub- 

 equilateralis, concentrice tenui-lirata ; umbonibus parum elevatis, 

 tumidis ; epidermide viridi-corneo, fusco-zonato ; plerumque C. 

 cornem similis. Long. |-; all. |- ; lat. ^ poll. Hob. New South 

 Wales ? 



It is larger, more rounded in outline, and more globose in 

 form than C. cornea. The epidermis is less glistening, of a 

 deeper green, and exhibits no traces of radiations. The liga- 

 ment is shorter and more prominent. The beaks, hinge, and 

 furrowing are nearly the same in both. 



Cyclas patella. T. parva, tenuis, rotundato-ovalis, lenticu- 

 laris, modice cavata, concentrice exiliter sulcata, epidermide 

 luteo-viridi induta ; umbonibus medianis, rotundatis, baud eleva- 

 tis : intus lactea : cardo dentibus duobus minutis cardinalibus 



