GEOLOGICAL FORMATIONS. 675 



flowing over the sloping beds of the formation has here a consider- 

 able fall, which has been utilized as a water power. 



Paleontology. The Lower Magnesian limestone is characterized 

 by the extreme rarity of its organic remains. Indeed, it is believed, 

 that up to the period of this survey, no fossils whatever have been 

 reported from the formation in this portion of the state. During the 

 summer of 1873, several places were discovered which establish their 

 existence beyond a doubt. The fossils are usually found imbedded in 

 the drusy quartz, with which the formation abounds, arid usually in 

 the form of casts. Some, however, have been found in the limestone. 



From several localities, the following are selected, because fossils 

 are more readily found there than elsewhere: 



(1) The S. W. qr. of the S. W. qr. of Sec. 4, T. 5, E. 5 E., on the 

 southwestern slope of the hill near the creek. 



(2) The S. W. qr. of Sec. 12, and the N. W. qr. of Sec. 13, T. 5, 

 R. 4 W. 



ST. PETERS SANDSTONE. 



Geological Boundaries. The formation known by the above title 

 in the geological reports of Io\va, Minnesota, Illinois and Wisconsin, 

 w r as formerly frequently designated as the Upper Sandstone, in contra- 

 distinction to the Lower, or Potsdam. It is thought best on account 

 of uniformity, to adhere to the present name. It is found in the val- 

 leys of the Grant, Platte and Pecatonica rivers, and their tributaries; 

 and in Green county, it forms the valley of the Sugar river and its 

 branches, this valley being in many places as wide as that of the Mis- 

 sissippi. North of the dividing ridge it is found about the head- 

 waters of the streams which flow into the Wisconsin, having its 

 northern outcrop usually within two or three miles of the river, and 

 as far east as Boscobel, forming a portion of the bluffs which inclose 

 the river valley. 



Lithological Characteristics. The formation differs from the 

 Potsdam sandstone, in that it contains no beds of limestone or shales 

 interstratified with it, but presents at any given locality a homogene- 

 ous structure through its entire thickness. Its color varies from snow 

 white, through all shades of yellow, to a very dark red, and in texture, 

 from friable crumbling sand, to compact and fine-grained stone. Be- 

 neath the microscope the particles of sand appear rounded and water- 

 worn. The same color and texture usually exist through the entire 

 thickness at any given place. 



The St. Peters formation frequently impresses upon the surface of 

 the country an appearance of terraces, although no true terraces, such 



