Clear distinction is recognized between divisions of
Pennsylvanian rocks in the midcontinent region that are based on
relation to boundaries having significance as markers of
geologic time, and those that are defined primarily by
lithologic characters. The boundaries of time-rock units, which
are determined primarily on the basis of sedimentation,
interruptions of sedimentation, and other features of historical
geology, are obscure but geographically extensive
disconformities that separate successions of Pennsylvanian
strata having different paleontological attributes. Greatest
change in the components of fossil faunas and floras closely
coincides with the horizons of these disconformities, which are
inferred, therefore, to denote relatively prolonged
interruptions in sedimentation. Erosion of previously
accumulated deposits, which is indicated at many places as
accompaniment of nondeposition, ranges from barely perceptible -
to quantitatively very considerable. The deposits occurring next
above the disconfor