THE CARBONIFEROUS SYSTEM. 103 



In Illinois the Lower Carboniferous formation is, for the most part, cal- 

 careous. It consists of — 



1. The Chester or Kaskaskia limestone. 



2. The St. Louis limestone 250 feet thick. 



3. The Warsaw limestone 50 to 100 



4. The Keokuk limestone 40 to 50 " 



5. The Burlington limestone 60 to 100 " 



All of which are underlaid by the " Kinderhook group," consisting of 

 shales and sandstones, with some local limestone beds having a thickness 

 of about 100 feet. The Illinois series contains many fossils which are 

 identical with those found in the Waverly of Ohio, and we have every 

 reason to believe that the beds which include them are the equivalents 

 of each other ; the lithological differences which they present being due 

 to the fact that the Waverly of Ohio is a shore deposit, while most of the 

 Lower Carboniferous of Illinois was laid down in an open sea. The reach 

 of this sea is indicated by the spread of its calcareous sediments ; and 

 since the Coal Measures of Michigan, Illinois, Kentucky, Tennessee, 

 Alabama, and Virginia are underlaid by this calcareous stratum, we have 

 in this evidence of a continuous ocean, which, during a portion of the 

 Lower Carboniferous period, occupied the area of the States that have 

 been enumerated, but which reached no farther (at least in sufficient 

 purity to form limestones) than central Ohio and the southern line of 

 Pennsylvania. 



It is also evident that the Lower Carboniferous period was one of pro- 

 gressive continental depression, for the lower rocks deposited in this period 

 over a great area are mechanical sediments, while the overlying calca- 

 reous deposits, thickest at the south and west, gradually thinning out 

 toward the north and east, are by their volume a measure of the length 

 of time during which, in any locality, marine conditions prevailed. 

 Hence we must suppose that the thin edge of the calcareous member of 

 the Lower Carboniferous series represents the uppermost, last formed 

 portion of the mass ; and this is more widely extended than the lower 

 beds, because, at the time of its deposition, the sea had further encroached 

 upon the land. 



THE CARBONIFEROUS CONGLOMERATE. 



In many parts of Europe, especially in England, and throughout most 

 of the area occupied by Carboniferous rocks in America, the middle por- 

 tion of the Carboniferous series is indicated by a mass of sandstone, of 

 greater or less thickness, containing vast numbers of quartz pebbles, and 

 forming a pudding-stone, or conglomerate. In England this rock is 



