24 The American Geologist. July, iss*^ 
In considering geological classifications for any region, 
great or small, it has come to be recognized of late years that 
no satisfactory arrangement can be established in accordance 
-with any one standard of comparison yet devised. It can 
only be accomplished as the final outcome of a comparison of 
all groups of pertinent data, by an interpretation of all the 
branches of the physical history taken etisemble. Applying 
these principles to the region under consideration and weigh- 
ing all the evidence derived from the ditf'erent lines of testi- 
mony a basis is believed to be found that will furnish a 
natural and satisfactory classification of the strata that is 
applicable throughout the extent of the Western Interior 
basin. In the Carboniferous of the region there are recogniz- 
able three distinct classes of sediments : (1) the marginal or 
coastal deposits, (2) the strata laid down in the more open 
sea, and (3) the off-shore depositions. These form four series, 
which are so arranged that the ott'-shore sediments form the 
top and the two maritime formations are separated hy the 
coastal deposits. 
The four series are sharply contrasted. The first comprises 
heavy-bedded limestones, rich in marine fossils, but with com- 
paratively little shale and scarcely any sandy material. The 
second series is characterized by rocks that are prevailingly 
sandy and clayey shales and sandstones with scarcely ixwj 
limestone. The individual beds rarely have a wide geograph- 
ical extent, but replace one another in rapid succession, both 
laterally and vertically. The fossils contained are largely 
brackish- water or shore species ; remains of pelagic organisms 
are not numerous. These and many other phenomena attest 
a constantly shifting shore-line and shallow water. The third 
class of beds is composed largely of calcareous shales, with 
heavy and numerous strata of limestone. The clay shales and 
sandstones are practically absent. The individual layers, es- 
pecially the limestones, are homogeneous and continuous over 
broad areas. The faunas are composed chiefly of the more 
strictly open sea forms. The foui'th series is more closely re- 
lated in its lithological features to the second, but the calca- 
reous beds are more numerous, particularly in the lower part, 
and all the layers have much greater continuity. In the latter 
respect it approaches the third class. 
