180 PROCEEDINGS OF THE NATIONAL MUSEUM. tol. 41. 



siliceous content of the water at the end of the subordinate periods. 

 The main beds of each period are formed of remarkably homogeneous 

 limestones, very regularly and evenly stratified, almost entirely com- 

 posed of the calcareous remains of countless myriads of crinoids, and 

 interrupted only by the siliceous invasions above mentioned, result- 

 ing in the deposition of the heavy cherty beds of passage between the 

 different members. It has a total thickness of upward of 200 feet 

 of solid limestones and chert, the latter being one-third or more of 

 the whole. The chert beds are not wholly barren, for thin bands or 

 lenses of limestone are found irregularly throughout them, often filled 

 with remains of crinoids that struggled through under the changed 

 environment. The coloration of the limestone beds is remarkably 

 uniform for each horizon. There is little muddy shale until rather 

 late in the Keokuk, when there were evidently considerable changes of 

 level, with migration into shallower waters to the eastward, resulting 

 in the deposit of the shale beds of the Crawfordsville region in Indiana. 



These facts indicate an unusually favorable condition for a suc- 

 cession of life during long periods, and for this reason the Mississippi 

 River region above indicated is taken as the typical one for this 

 formation, with which other occurrences of any of its members should 

 be compared, when considering their faunal or stratigraphic rela- 

 tions. This was not the case with the occurrences in southern 

 Indiana and Kentucky, where, as already pointed out, there must 

 have been frequent changes of level and of chemical constituents, 

 resulting in much irregularity and interruption in the deposition of 

 the strata and variability in their fossil contents. There was 

 evidently a strong migration of Keokuk species to the southward, 

 for there is a more definite assemblage of typical forms in Kentucky 

 and Tennessee than in the central Indiana region. 



The boundary between the two Burlington beds is really more 

 sharply marked than that between the Burlington and Keokuk, 

 there being a much more gradual transition of forms between the 

 latter, and the passage beds being more fossiliferous. Between the 

 Lower Burlington and the beds preceding it there is no such definite 

 line of separation. 



In White's original section at Burlington ^ the top of the Kinder-, 

 hook — his bed No. 7 — is shown to be an impure limestone, some- 

 times magnesian, which passes insensibly into the Lower Burlington 

 Limestone. It is thin at that point, but thickens to the northwest 

 to 40 feet of limestone containing the rich fossil beds of LeGrand, 

 in Marshall County; and to the southward it also thickens to upward 

 of 100 feet in Missouri, where it takes the name in part of the Loui- 

 siana and in part the Choteau Limestone; and occasional highly 

 fossiliferous beds of shale, like those at Fern Glen, may constitute a 



1 Geology of Iowa, vol. 1, 1870, p. 193. 



