174 GEKEEAL GEOLOGY. 



formation in Iowa are, so far as known, a few traces of the 

 stems of Crinoids fonnd near McGregor. Of course, no 

 specific identification of them could be made. Some traces 

 were also observed there which are doubtfully referred to 

 fucoids. 



ST. PETER'S SANDSTONE. 



Area and General Characters. This formation is remark- 

 ably nniform in thickness throughout its known geographical 

 extent, and also in lithological characters. It is a clean grit, 

 light colored, very friable rock; so pure in its silicious 

 composition that it is probable some portions of it may be 

 found suitable for the manufacture of glass. Usually, how- 

 ever, it is more or less colored with oxide of iron, and a 

 couple of miles below McGregor it is much colored by various 

 shades of red and yellow, where it takes the local name of 

 " Pictured Rocks." It has been exposed principally by the 

 same fluvatile erosion that brought the Lower Magnesian 

 limestone and Potsdam sandstone to view ; but it is evident 

 that it occupies the surface of a large part of the northern 

 half of Allamakee county, immediately beneath the drift. 



It is very seldom hard enough to be used as stone, and 

 unless it should prove suitable for glass in some places, it is 

 probably of no economic value whatever. It contains no 

 fossils, and is referred to the Primordial group upon litho- 

 logical and stratigraphical grounds. 



TEENTON GEOUP. 



THE TRENTON LIMESTONE. 



Area and General Characters. With the exception of this, 

 all the limestones of both Upper and Lower Silurian age in 

 Iowa, are magnesian limestones — nearly pure dolomites. The 

 rocks of this formation also contain much magnesia, but a 

 large part of it is composed of bluish compact common lime- 

 stone. The surface occupied by the Trenton limestone is 



