118 



stop to sedimentation in that portion of the basin during all 

 of the remaining- Onondaga time. In the vicinity of Grand 

 Tower, and in the north part of Union county, however, de- 

 position was uninterrupted, and there was laid down above 

 the sandstone a thickness of 125 feet of Onondaga limestone. 



The transition beds from the sandstone to the overlying 

 limestone, and all of the limestone member, are well exposed 

 in what is known as the Devil's Backbone and Bake-oven 

 ridge, one-half mile to one mile north of Grand Tower. The 

 contact of the Grand Tower limestone with the succeeding 

 Hamilton beds may also be clearly seen in the Backbone 

 bluff. 



The strata in this region are all inclined to the eastward at 

 an angle of about 24 degrees. This is due to the upthrow on 

 the east side of a north-south fault plane that cuts the strata 

 some distance further west, in Missouri. 



Method of Work. In the detailed study of the strata of 

 the Grand Tower formation, and efifort was made to obtain as 

 nearly as possible the entire fauna of the vorious rock layers, 

 and also to determine the vertical range and relative aboundance 

 of the dififerent species of fossils. To this end the exposed rock 

 ledges were arbitraril divided into zones from six inches to only 

 a few feet in thickness. The fossils from each of these layers of 

 zones were kept separate, in order that the range and the rela- 

 tions of the successive faunules might be ascertained. This 

 method of work has made it possible to determine the species 

 of fossils that are distinctive respectively of the various hori- 

 zons of the formation, and has furnished much information 

 concerning the fauna that could be obtained in no other way. 



Detailed Sections. There are given below detailed sections 

 of the Grand Tower strata exposed in Illinois, accompained by 

 lists of fossils that were collected from the successive horizons. 

 In these lists the relative abundance of each species is indicated 

 by the suffix r ( = rare), c (^common), or a (^.abundant) after 

 the name. 



Section of the basal portion of the Grand Tower formation 

 exposed along a small stream in the northwest quarter of sec- 



