CARBONIFEROUS AGE. 305 



Devonian still lingered ; and fragmental deposits, either clayey or sandy, 

 were made over the Mississippi region, as well as to the eastward. 

 With its progress, the crinoidal sea increased in depth and in freedom 

 from sediments ; yet these continued, at intervals, through the forma- 

 tion of the Kinderhook beds, though to a less extent in Missouri than 

 farther north. The earthy depositions then became less frequent, the 

 rock of the Burlington and Keokuk group being mainly limestone ; 

 but, at the same time, as remarked by Hall, the northern border of 

 the Interior sea had moved southward, the northern limit of the Bur- 

 lington limestone being two hundred miles farther south than that of 

 the earlier beds, and that of the Keokuk and St. Louis group still far- 

 ther south. This limestone-making sea, though gradually deepening 

 in the valley, did not entirely preserve its freedom from sediments, far 

 east of Illinois ; for even central Tennessee and Ohio, as well as the 

 Appalachian region, was contemporaneously a region of accumulating 

 sand and gravel beds, and probably for the most part one of shallow 

 waters. During the progress of the St. Louis epoch, the sea deepened 

 in Tennessee, and some limestones were made, from Crinoids and 

 shells ; and moreover, according to C. A. White, it extended north- 

 ward, in Iowa, nearly to the limit of the Kinderhook group. After- 

 ward, there was again a contraction on the north, the Chester lime- 

 stones reaching only to Alton, Illinois ; but in other directions the 

 sea had then greatly widened limits and increased depth, the lime- 

 stone spreading to the southward, through Tennessee and Kentucky 

 to West Virginia, Alabama, and Mississippi, and being represented by 

 thin beds in Ohio and western Pennsylvania. In the Appalachian 

 region, there were not only fragmental beds, but a very great thick- 

 ness of them, the thickness increasing from the New York boundary 

 on the north and from wester a Pennsylvania on the west, toward the 

 region of Pottsville, where the whole was 4,000 to 5,000 feet, proof that, 

 along the central portions of the region, there was this amount of sub- 

 sidence during the period, and that the State of New York on the 

 north did not participate in it, as it had done in the preceding Catskill 

 period. This thickness of Subcarboniferous rocks is four times that in 

 the Mississippi valley. 



The region of the Cincinnati geanticlinal, from Lake Erie into Ken- 

 tucky, was, as stated by Newberry, a peninsula during the era. 



Michigan was to some extent independent in its movements, and yet 

 there, as elsewhere, the latter part of the period was the time of lime- 

 stone-making, and therefore of clearer waters. This was true also of 

 the Carboniferous region of Nova Scotia and New Brunswick, where 

 the beds are mainly fragmental. 



The chert, which abounds in some of the beds, probably has the 

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