History of the Missouri Paleozoic. — Broadhead. 885 
down sufficiently to receive 3,000 feet of sediment in the Ap- 
palachians and New York, but this thinned out as above 
stated to 200 feet in southeastern Missouri, becoming zero in 
Callaway county. Westwardly dry land continued. The 
subsidence continued so as to receive about 100 feet of Devo- 
nian limestone in northern Missouri, thinning out in Pettis 
county. The Devonian sea extended northwardly to Wiscon- 
sin and around the eastern slopes of the Ozarks and beyond 
the Appalachians. Upon the Ozarks, and extending far west, 
dry land still prevailed. At the close of the Devonian on the 
east and south a clay deposit accumulated in certain shallow 
estuaries, which later by the absorption of certain hydrocar- 
bons became a black slate. The Devonian period in Kentucky 
and in a portion of Missouri, notably at the falls of Ohio and 
in Callaway county, Mo., was characterized by extensive coral 
reefs. The Upper Silurian and Devonian in the eastern 
states were also rich in coral forms. 
After the Devonian was laid down there seems to have been 
a marked dynamic change. At the close of the Devonian 
there was a subsidence of most of the Mississippi valley west 
of the Mississippi river, with the exception of Minnesota and 
part of Iowa, and the Archaean of southeastern Missouri and 
of Texas. The Ozarks subsided sufficiently for the early Car- 
boniferous seas to cover their margins. This was the begin- 
ning of the Subcarboniferous. On the east and north these 
beds rest upon the Devonian, but on the west they repose 
upon the Ozark series. The several groups of the Subcarbon- 
iferous, as the Paleozoic sea sank, continued to gradually ac- 
cumulate. First the Chouteau and its several formations, 
next the Burlington with its rich crinoidal fauna and inter- 
calated chert, then the Keokuk, and then the St. Louis, — each 
to be afterward in a great measure eroded. That there was a 
slight subsidence and then an elevation of the Ozarks after 
the Burlington had accumulated is shown by the occasional 
finding of patches of limestone of that terrane along the 
northern and western front and resting upon the Ozark scries, 
indicating that the erosion of the early Subcarboniferous muBl 
have been great. Previous to this, the erosion of the later 
Ozark series must also have been great, but very probably it 
was largely aerial. Since the close of the Burlington the 
