OVERLAPPING STRATA. 
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of greenish and whitish marls of the Cretaceous formation. 
The lowest, and therefore the oldest, bed of the horizontal 
series is usually the sand and conglomerate;, a, in which are 
rounded fragments of stone, from an inch to two feet in di¬ 
ameter. These fragments have often adhering shells attach¬ 
ed to them, and have been bored by perforating mollusca. 
The solid surface of the inferior limestone has also been 
bored, so as to exhibit cylindrical and pear-shaped cavities, 
as at c, the work of saxicavous mollusca; and many rents, as 
at which descend several feet or yards into the limestone, 
have been filled with sand and shells, similar to those in the 
stratum a. 
Overlapping Strata. —Strata are said to overlap when an 
upper bed extends beyond the limits of a lower one. This 
may be produced in various ways; as, for example, when al¬ 
terations of physical geography cause the arms of a river or 
channels of discharge to vary, so that sediment brought down 
is deposited over a wider area than before, or when the sea- 
bottom has been raised up and again depressed without dis¬ 
turbing the horizontal position of the strata. In this case 
the newer strata may rest for the most part conformably on 
the older, but, extending farther, pass over their edges. Ev¬ 
ery intermediate state between unconformable and over-lap¬ 
ping beds may occur, because there may be every gradation 
between a slight derangement of position, and a considerable 
disturbance and denudation of the older formation before 
the newer beds come on. 
