[Reports volI-XIII] . ges in that particular zoologicalgroup in which such extreme developments take place. It is apparent from a close study of the crinoids, and in asomewhat less marked degree, perhaps, among other zoologicalgroups, that there was an abrupt change of physical conditionsat the close of the Keokuk epoch. For, at this time, more thanone-half of the Carboniferous genera of this class had becomesuddenly extinct. As already stated in another place, the abrupt extinctionof a large proportion of the criuoidal and other forms of life atthe close of the Keokuk is certainly suggestive
[Reports volI-XIII] . ges in that particular zoologicalgroup in which such extreme developments take place. It is apparent from a close study of the crinoids, and in asomewhat less marked degree, perhaps, among other zoologicalgroups, that there was an abrupt change of physical conditionsat the close of the Keokuk epoch. For, at this time, more thanone-half of the Carboniferous genera of this class had becomesuddenly extinct. As already stated in another place, the abrupt extinctionof a large proportion of the criuoidal and other forms of life atthe close of the Keokuk is certainly suggestive of a series ofwide-spread changes in the geographic and bathymetric extent ofthe great interior sea; and there is sufficient evidence to in-dicate that at the close of the Keokuk, the northern coast lineof the broad shallow gulf which occupied the area moved • Jonr. Boston Soo. Nat, Hist., Vol. VII, pp. : Amer. Jour. Soi., (3), Vol. XXXVIII, pp. 191-192. 1889. MISSOURI GEOLOGICAL SURVEY. PLATE LOVERS LEAP, HANNIBAL. DISTIflBUTION OF THE BURLINGTON 6 1 rapidly southward, and this movement was soon followed byslight depression. The Saint Louis waters then pushed north-ward again, in some places several hundred miles. Burlington Limestone—The lithological characters of theBurlington limestone are remarkably constant over broadstretches of territory. At the original locality it is a coarse-grained encrinital limerock; hard, compact and heavily beddedin some layers, porous in others, with scarcely enough of firmand cementing material to hold the crinoidal remains some places, however, it is very compact, fine-grained andearthy, and is then reddish or deep brownish in color. Ohertnodules abound locally. The lower portions are usually muchmore heavily bedded than the upper— the partings of the in-ferior beds being commonly a coarse, calcareous reddish sand,while in the upper strata, clay seams are not infrequent. In western Illinois, in Pike a
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