CAEBOKEFEROUS SYSTEM. 211 



eighty miles below Keokuk, presenting there precisely the 

 same characters that it has in Iowa. 



The calcareous portion of the formation — the Keokuk lime- 

 stone proper — consists of more or less massive layers of sub- 

 crystaline limestone, alternating with layers of bluish marly 

 shale, sometimes amounting only to a mertj^parting between 

 the layers of limestone, but at other times they form beds of 

 two or three feet in thickness. This is its character at and in 

 the vicinity of Keokuk, but as it thins out to the northward it 

 is less shaly. The upper silicious portion of the formation, 

 known as the Geode bed, nowhere reaches so great a thickness 

 as the upper silicious portion of the upper division of the 

 Burlington limestone does at the rapids above Keokuk ; but 

 the great thickness there of the latter is probably only a local 

 development. If that is the case, we have a gradual increase 

 in bulk of these silicious beds of passage from that one which 

 separates the two divisions of Burlington limestone, to the 

 Geode bed inclusive. Not only is this probably the case, but 

 there is also a gradually increasing amount of silicious 

 matter disseminated throughout the limestone itself, from the 

 lower Burlington limestone to the Keokuk limestone inclusive. 

 This silicious matter in the limestones has not been mechan- 

 ically deposited as sand has, yet much of it is in a finely 

 divided condition. It has a tendency to form irregular con- 

 cretions in the limestone, sometimes accumulating around a 

 fossil shell as a nucleus, or the shell itself becomes silicious 

 while the imbedding limestone is quite pure, showing that 

 there has been a movement and re- arrangement of the 

 molecules of silex since the original deposition of the 

 material now composing the rock. With the completion 

 of the Geode bed above the limestone portion of the forma- 

 tion, the tendency to the accumulation of silicious strata by 

 precipitation of silex is very greatly diminished. The St. 

 Louis limestone is not free from cherty bands and nodules, 

 but their aggregate amount is so small that it presents quite 

 a contrast with the formations which preceded it. 



The geodes of the Geode bed are so attractive, as showy 



