60 GEOLOGICAL FORMATIONS 



limestone" in a variety of senses — sometimes referring to the 

 Burlington alone, sometimes to both Burlington and Keokuk, 

 and often to the Burlington and a part of the Keokuk. Partly 

 on lithological grounds, but chiefly for paleontological rea- 

 sons, the Augusta may be regarded as made up of three mem- 

 bers — upper, middle and lower — coinciding essentially with the 

 Keokuk and the upper and lower Burlington limestones. In 

 regard to the fossils of the three horizons, the most conspicu- 

 ous general differences were first suggested by White*, and 

 quite recently t they have received further attention. These 

 differences may be reiterated here. Those species from the 

 lower Burlington are of small size, delicately constructed and 

 highly ornamented. In the upper division of the Burlington 

 the peculiar delicacy so characteristic of the forms of the 

 lower bed is absent or has assumed a ruder phase, while in the 

 Keokuk the crinoids are notable for large size, rough and mas- 

 sive construction, bold and rugged ornamentation, and a con- 

 spicuous exaggeration in many structural details. The last 

 named consideration is of great interest, since it appears that 

 in general the exaggeration of anatomical features is indicative 

 of important biologic changes in that particular zoological 

 group in which such extreme developments take place. 



It is apparent from a close study of the crinoids, and in a 

 somewhat less marked degree, peihaps, among other zoological 

 groups, that there was an abrupt change of physical conditions 

 at the close of the Keokuk epoch. For, at this time, more than 

 one-half of the Carboniferous genera of this class had become 

 suddenly extinct. 



As already stated in another place, the abrupt extinction 

 of a large proportion of the criuoidal and other forms of life at 

 the close of the Keokuk is certainly suggestive of a series of 

 wide-spread changes in the geographic and bathymetric extent of 

 the great interior sea; and there is sufficient evidence to in- 

 dicate that at the close of the Keokuk, the northern coast line 

 of the broad shallow gulf which occupied the area moved 



* Jour. Boston Soc. Nat. Hist., Vol. VII, pp. 224-225. 1860. 

 tKeyes: Amer. Jour. Sci., (3), Vol. XXXVIII, pp. 191-192. 1889. 



