OF THE NORTHWESTERN STATES. 17 



decidiosa, 15 to 30 feet below the surface, same elevation as above. Also a frag- 

 ment Of Planorbis in red clay, 60 feet above the river, and 580 to 700 feet above 

 the ocean. 



Near Peoria, Illinois. — Helix concava, Helix chersina, in coarse sand and gravel 

 beds, 120 feet above Illinois river. 



Seven Miles East of St. Louis, Mo., River Bluff. — Helix alternata, Helix stria- 



tella, Helicina 1 Amnicola 1 Lymnea valvata, Succinea obliqua, in 



Loess-like loam; with calcareous concretions, 30 to 40 feet below surface, 150 to 

 200 feet above low water in the Mississippi river and 500 to 580 feet above tide. 



New Harmony, Indiana. — Helix hirsuta, Helix fraterna. Helix minuta. Pupa 



% (New Species) Amnicola? Lymnea decidiosa, in Loess-like loam 50 to 100 



feet above the Wabash river. 



For the names of these shells I am indebted to Prof. Agassiz and Dr. J. S. 

 Newberry. 



Ancient Terraces and Ridges, 



Throughout the western country there are bluffs and terraces, which are com-^ 

 posed of solid rock, and which are due to geological causes, more ancient than 

 those under discussion. In some cases the boldness of rocky terraces has been 

 toned down by the drift forces, as well as by the disintegrating power of the 

 atmosphere. 



Terraces due to drift action are composed of boulders, gravel, hardpan, and clay 

 or sand. In most cases they represent an ancient shore. During the period oi 

 emergence, where the water line remained fixed long enough to allow the waves to 

 cut into the slopes of the shore, a steep bluff was the necessary result. 



The following plan aijd profile at Cleveland, Ohio (on page 26), illustrate the 

 changes that have taken place there in soft strata by wave action alone. If the clay 

 bluffs on the present shores of the lake were no longer undermined by the waves, 

 terraces would be formed. Suppose by a depression in the outlet of Lake Erie, its 

 surface should fall rapidly thirty feet. The present shore line would forever mark the 

 present level of its waters. An ancient shore can be traced, in the form of a terrace, 

 on the south shore of this lake, from near Erie, in Pennsylvania, to the Vermilion 

 river, in Ohio, a distance of about one hundred and twenty-five miles. It is nearly 

 parallel to the coast, and its base is about one hundred and sixty feet above water 

 level. On Lake Superior there are many such terraces well defined. 



The most conspicuous of these may be seen on the highlands, southwest of Bay- 

 field, Lake Superior, opposite the Apostle Islands. Here there are four, the lowest 

 of which has its upper surface from 100 to 120 feet above water level, and the 

 fourth or highest 400 to 430 feet. 



What are commonly known as " Lake ridges," are, it seems to me, not due to 

 the same cause as the terraces. They are not ancient beaches, but the result of 

 lateral cuiTents such as in all waters cause subaqueous bars and spits, rudely parallel 

 with the shore. Their composition is universally coarse water-washed sand, and 

 fine gravel. Around Lake Ontario on the Canada side. Sir Charles Lyell and Mr. 

 Ray found traces of eleven of these ridges more or less parallel with the present 



3 May, 1866. 



