292 C. R. KEYES THE PRINCIPAL MISSISSIPPI AN SECTION. 



Burlington <iAmestone. — The lithological characters of the Burlington arc 



remarkably constant over broad stretches of territory, and they are practi- 

 cally identical over its entire extent, so far as it has been traced accu- 

 rately, from northern-central Iowa to western Illinois, southwestern Mis- 

 souri and Arkansas. Everywhere it is the same coarse grained encrinital 

 limestone, intensely white and quite pure in certain layers. For the most 

 part its geographic distribution is west of the Mississippi river. East of 

 the stream the typical exposures of this rock are unimportant and un- 

 known beyond the immediate vicinity of the great watercourse. 



Keokuk Limestone. — The upper member of the Osage, on the other hand, 

 has its distribution chiefly on the eastern side of the " Father of Waters."' 

 c( tvering a wide area in Illin< >is, Indiana, Kentucky and Tennessee. West 

 of the river the most typical exposures are in southeastern Iowa and 

 northeastern Missouri. At Boonville, in central Missouri, where these 

 rocks have been reported, the faunas contained do not indicate the true 

 Keokuk. In the southwestern part of the same state no typical Keokuk 

 lias been observed, so far as is known. The encrinital limestone of that 

 region, which has been thought by some to represent both the Keokuk 

 and Burlington limestones of the more northern localities, appears to be 

 the latter alone. Extensive collections of fossils made in various parts 

 of the formation show few species that can 'be regarded as belonging^*) 

 the true Keokuk. This is all the more remarkable from the fact that a 

 vertical section of the Kinderhook and Burlington beds of the region is 

 essentially identical, lithologically, with the one of northeastern Missouri. 

 After all, the upper member may be present, for the recent personal 

 observations were not conclusive enough to preclude its existence entirely. 



There is, however, another very suggestive consideration bearing upon 

 the relations of the Keokuk and Burlington limestones which is worthy 

 of notice. It was strongly impressed some years ago while engaged in a 

 study of the Carl toniferous echinoderms of the Mississippi 1 >asin. Accord- 

 ing to this inference it appears that the lower portions of the Keokuk and 

 Burlington rocks were deposited nearly at the same time but in practi- 

 cally separate basins, the barrier being approximately along the line of 

 the present Mississippi river. As the obstruction Avas gradually removed, 

 the animal forms of the two districts mingled more or less completely, 

 and those of the eastern area being better adapted to the changing condi- 

 tions displaced the old occupants of the eastern portion of the Burlington 

 territory as the sea became gradually deeper, eventually replacing them 

 altogether; so that in the area of the typical localities of these rocks a 

 succession of faunas is represented that is not shown elsewhere 1 . Thus 

 the so-called Keokuk overlapped, by degrees, the Burlington, and while 

 the fauna of the upper Keokuk was living where portions of Iowa and 



