224 GENERAL GEOLOGY. 



largely composed of magnesian limestones, the proportion 

 diminishing in those of Devonian age. Coming next to the 

 Sub-carboniferous rocks we find that magnesia enters largely 

 into their composition also, but in a still further diminished 

 proportion. Magnesian limestone forms a considerable 

 proportion of the Kinderhook beds; less in the Lower Bur- 

 lington limestone ; it is almost absent in the Upper Burling- 

 ton limestone; and also in the Keokuk limestone proper, 

 although the Geode bed contains some magnesia. In the 

 lower division of the St. Louis limestone there are important 

 beds of magnesian limestone again. With the completion of 

 the last named formation the production of the magnesian 

 limestone seems to have ceased among the rocks of Iowa, for 

 numerous analyses of Iowa rocks of later date fail to show 

 more than an insignificant amount of magnesia in their com- 

 position. 



Again, at the commencement of the Kinderhook epoch 

 there was a considerable deposition of argillaceous material. 

 Afterwards, and during the whole epoch of the Burlington 

 limestone there was very little of such material deposited. 



In the Keokuk limestone formation again we find important 

 beds of bluish, shaly clays, and similar material also 

 prevails in the St. Louis limestone, both as beds of consid- 

 erable thickness, and as marly partings between the layers of 

 limestone. 



The remains of fishes and echinoderms constitute such 

 prominent features in the fossil fauna of the Burlington and 

 Keokuk limestones, as to impress that prominence upon the 

 palaeontology of the whole group. Although the Devonian 

 age has been called the age of fishes, and perhaps very 

 properly so as relates to the geology of the whole earth, 

 yet so far as Iowa is concerned the rocks of no period can 

 compare with the Sub-carboniferous in the abundance and 

 variety of its fish remains. 



Crinoids and other echinoderms are found in the strata 

 of all ages, but they reached an extraordinary degree of 

 profusion in the seas of the Sub-carboniferous period, as 



