262 GEOLOGY OF EASTERN WISCONSIN. 



mosia to be associated with Lingulepis pinniformis, which is con- 

 sidered by Owen and Hall as characterizing the lower beds of the Pots- 

 dam, and Dicellocephalus Pepin ens-is was found with Illaenurus 

 quadrettus, which Prof. Hall refers to the middle Potsdam, thus show- 

 ing specific paleontological bonds with the lower strata. 



On the other hand there is not, so far as I am aware, any case of 

 the occurrence of any of these species in the Lower Magnesian lime- 

 stone, as limited in this report, there being ample reasons for believ- 

 ing that in all cases where Dicellocephalus Minnesotensis has been 

 doubtfully referred to this horizon, the specimens really belonged to 

 the Mendota or other intercalated beds. 



The lower divisions, constituting the main body of the Potsdam 

 sandstone, present but very few exposures in this region, and these of 

 very limited extent, so that we are dependent chiefly on the evidence 

 derived from Artesian wells for positive knowledge of their nature. 

 So far as thus indicated, there are four subdivisions. 



j. That which lies next below the Mendota beds is a light colored, 

 fine to medium grained sandstone. The constituent grains are chiefly 

 quartz, with a less quantity of chert and a few limestone and granitic 

 particles in some portions. It is slightly calcareous, even where the 

 limestone particles are not observable. 



4. The next division consists of a bluish green shale of highly cal- 

 careous nature, containing minute, glistening, mica-like scales. It 

 was not seen in outcrop, and may be quite local, though it appears 

 to be represented in the northwestern part of the state. 



5. The fifth division is very similar in nature to that above the last, 

 being a rather fine grained, light colored quartzose sandstone, contain- 

 ing occasionally some clay-like calcareous matter. 



6. The lowest subdivision is a very coarse, non-calcareous sand- 

 stone, composed of large grains of transparent, light colored quartz of 

 irregular but rounded form. 



Organic IZemains. The following is a list of the fossils found in 

 the Potsdam in this portion of the state. These, and all subsequent 

 identifications in this report were made by Prof. R. P. Whitfield, 

 whose eminent qualifications are a guaranty of their correctness: 



PLANTS. 

 Paleophycus. Sp. und. 



MOLLUSCA. BRACHIOPODA. 

 Orthis Pepina. 



AETICULATA. ANNELIDA. 

 Tubes of scolithus? 



