3 
common to the two habitats and one of these is said to be "so 
modified to meet surf conditions that ordinarily the specimens 
from the two localities would not be recognized as belonging to 
the same species.” This example of faunal variation obtaining 
over a very limited area emphasizes what may be expected in 
shallow seas as extensive as those of the past with their sedi- 
ments coming from many directions and derived from different 
sources, and with shores of the irregular character typical of 
bodies of water resulting from submergence of land areas. 
A somewhat deep-rooted — perhaps sub-conscious — feeling 
prevails that in many of the ancient seas, plants and animals 
were universally distributed and that the conditions of sedi- 
mentation were the same throughout, with the result that form- 
ations and members of formations are now continuous — with 
similar lithic and faunal characters — over the entire area in 
which deposition occurred. This theory, which assumes there 
was nothing different in one place from any other place is, to 
the writer, impossible. The tendency of those who adopt the 
theory of uniform sedimentation is to consider variations in 
lithology as evidence of distinct formations which bear a vertical 
and not a horizontal relation to each other. If, in addition to 
the lithological difference, there be a variation in faunal content 
the evidence is considered conclusive. Strata of similar lithology 
which lie on the same stratigraphic horizon, with slight differ- 
ences in their faunas, are also commonly interpreted as distinct 
formations. In many cases such conclusions may be correct, 
but the matter of lateral variation should be carefully con- 
sidered and fairly tested. 
Lateral gradation of sediments and faunas may so develop 
that one type of sediment with its fauna may overlap another — 
the conditions responsible for one type of deposition migrating 
laterally with respect to the other. The common interpretation 
would be “overlap” of the one by the other, a withdrawal of the 
sea, a land interval, and the development of an unconformity. 
A reverse migration of the conditions of deposition would bring 
about “overlap” from the opposite direction and another land 
interval with the resultant unconformity. The evidence would 
