Standard for American Carboniferous. — Keyes 303 
In the Mississippi valley section, the dividing lines between 
the several series are horizons which are probably capable of 
being very much more widely traced than has yet been at- 
tempted. Most of the planes separating the series are natural 
stratigraphid horizons. They are located ait levels where 
marked changes in sedimentation have taken place, where new 
phases of geological history begin, where lithological differ- 
ences were brought into strong contrast. Decided orogenic 
changes of a rather abrupt nature took place at the times rep- 
resented by these boundary' lines of the series. 
The subdivisions of the series are of minor importance in 
the present connection. Whatever mieht be these values in 
the ]\Iississippi valley they would necessarily have few or no 
elements of exact comparison with those of other geological 
provinces. 
The attempts to parallel the Mississippi valley section of 
the Carboniferous with that of Pennsvlvania have been not 
only uniformly unsatisfactory, but on the whole manifestly 
more or less fanciful. With the Mississippi section now so 
well determined, with its subdivision meaning something more 
than mere convenient groupings of strata, and with the ex- 
tension of the scheme westward, the Ohio and Pennsylvania 
sections come to have a significance that was wholly impossible 
to ascribe to them before. Of course with the present nomen- 
clature and present divisional lines the terranes can be com- 
pared only as approximate equivalents. 
Graphically represented, the serial terranes of the Carbon- 
iferous system! have, according to the best information we 
have, sonijething of the following relationships and develop- 
ments, shown in the subjoined cut (Fig. i) : 
ffocf^r Mii. 
Affl^nufiCHi^NS 
MiSbiSilPO, Vallhi 
^mh. 
Fig. 1. Relationships of the Carboniferous Series in America. 
In the East and in the West the base of the Carboniferous 
system appears to be well defined on account of the presence of 
