Postelsia 
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Sy 
CO 
that the conglomerate or the shale forms the 
surface rock, the strata overlying having been 
eroded. Plate XX. fig. 2. shows the heavily wooded 
cliff known as“ The Point.” Lhe horizemtal 
rock forming the floor leading to the sea is 
graywacke. 
In, all classes of rock are depressions of 
varying shape and depth, sometimes exposed 
by the tide, and” sometimes» (covercdas siitey 
possess characteristic fauna and flora, and are 
called tide-pools. The typical pools lie in the 
sandstone, because of the extreme softness of 
the rock, which renders the work of erosion 
easy. They are also abundant in shale, but 
are few in number in conglomerate. 
The pools in sandstone occur generally in 
strata which are nearly horizontal, so that the 
pool resembles a depression in a floor. Some- 
times the lower part of the pool has been worn 
away more than the upper part, leaving the 
uneroded strata as a projecting rock shelf, 
which affords a hiding place for sea-urchins, 
star fish, and other animals. 
The greatest number of small pools was 
