256 The American Geologist. October, looi, 
containing deposit was made at an age closely corresponding to the 
Clinton of the New York sections, and was incident to the deep- 
ening seas which soon after received the limestone formation of the 
St. Clair (Niagara) epoch. 
The Devonian of the Ozark region has always been a puzzle to 
geologists. Professor Williams' observations go a long ways In 
solving some of the most perplexing phases. "In numerous places 
in north Arkansas the evidences of an tmconformity separating the 
Silurian from the overlying Carboniferous, are very clear. In some 
cases there is no rock-material separating these two grand terranes. 
In other cases there are greenish shales, or coarse sandstones, with 
polished grains and rounded nodules of black shale ; and in the west- 
ern section the interval is occupied, in part, by a black shale, the 
Eureka shale of the Washington County report." The fauna in the 
fine shales which succeed the black shales is correlated with that of 
the Louisiana or lithographic limestone, and "is thus as late as the 
Kinderhook stage of the Eocarboniferous." 
The interpretation of the facts is "that the typical interval-mater- 
ials, the green shale and the Sylamore sandstone, were deposited after 
the period of the formation of the typical black shales which, along 
the borders of the Ozark uplift, was terminated, or actually driven 
outward, by the elevation of that region; that these particular de- 
posits mark the stage of sinking again of the land and the resultant 
erosion which introduced the Carboniferous formations for this re- 
gion ; that the time was at the very close of the Devonian and be- 
ginning of the Carboniferous eras. I conclude that the explanation of 
the varying age and nature of these deposits is due to the sections 
having been taken at places at lower or higher position on the grad- 
ually sinking land and expressing the overlap of the successively more 
recent deposits. Further study of the whole problem of the deposits 
filling the Devonian interval in the South has led to the conclusion 
that, however much erosion of the underlying Silurian formation took 
place, the sediments of black mud forming the shale, did not begin 
till after the beginning of the Devonian era. The age of the begin- 
ning of the new sedimentation being determined by the first fossils 
above the abrupt change, the unconformity may not be indicated by 
conspicuous modification of the plane of sedimentation. We should 
less expect real unconformity in the central part of the continental 
mass, as in the Mississippi valley region than on the borders where 
the folding and faulting has been chiefly concentrated." 
There is one statement concerning the Carboniferous which de- 
serves more than passing notice. It relates to the Spring Creek di- 
vision, which may be the equivalent in part of the Fayetteville shale 
of the western part of the state. "It is of interest to note the con- 
nection between the sharp and decided change of fauna and the change 
in the lithology of the rocks. The passage, beginning with the red 
marble, is from argillaceous shales, through calcareous and often 
crystallized, to cherty limestone, becoming more and more cTierty at 
