304 The American Geologist. November, looi. 
marked unconformities. The cle])ositional equivalents of these 
great erosional hrcaks in sedimentation have never heen sug- 
gested or sought for. What thev represent in terranes would 
be of greater interest to know. They may have been formed 
while verv considerable deposits were being laid down as Car- 
boniferous formations. The horizon of unconformity might 
then be separated from the real base of the Carboniferous by a 
stratigraphical interval of very considerable extent. As an 
analogous illustration of what vsst possibilities might occur, 
even in the Carboniferous system itself, the Arkansan series of 
the Ozark region may be cited.* 
In the Mississippi valley there anpear to be no indications 
of the existence of unconformable relationships between the 
beds called Devonian and those termed Carboniferous. The suc- 
cession so far as is now known is unbroken and the sediments 
are of marine origin. For this reason a Devonian facies of the 
fossils may have been retained in this region long after the 
Carboniferous was fully inaugurated in other provinces. 
The problems which arise for solution in this connection 
are many ; and their definite formiilaticu is of fully as much 
importance as the recognition of the fact that they exist. 
As a terrane, or structural unit the typical Alississippian 
series can be probably never exactlv extended into the eastern 
provinces or into the western districts. In the first named direc- 
tion terranes of very different lithological com-Dosition replace 
it. In the second direction there must be for a long time yet 
difficulty in separating it from lithologically similar forma- 
tions above, through the whole sequence of which the succes- 
sion of faunas is unbroken and must change very gradually. 
.Just how much of the Pocono and Maunch Chunk forma- 
tions of the Pennsylvania section which are commonly re- 
garded as forming the Lower Carboniferous, is the strati- 
graphic equivalent of the Mississippian must depend upon 
some further exact work of correlation in the region occupied 
by , Kentucky and Indiana. The sajnte is true, to a less de- 
gree, of the Ohio section. At present, it seems probable that 
a part of the Waverly will be found to belong more properly 
to the Devonian. 
In the east, the Mississippian appears to be replaced by at 
least two terranes of as yet indeterminable vertical extent 
•Bw/7. Geot. Soc. America, vol. xii, 1901, p. 179. 
