302 Postelsia 
causes is of course the influence of plants and 
animals, which have acquired a position in the 
pool. 
The question may arise as to the cause of 
the removal of the calcite which acts as, Wa 
cement between the sandstone and the con- 
cretion. It is well known that as fresh water 
flows over the land, it absorbs carbon 
dioxide from the air, forming carbonic acid. 
This acid, though weak, is strong enough to 
cause large quantities of calcium carbonate, 
usually insoluble, to dissolve in the water. Sea 
water contains a large amount of carbon diox- 
ide, derived partly from the inflow of fresh 
water,and partly from the decay of plants and 
animals. Twice a day it flows over the con- 
cretions, and because of the presence of carbonic 
acid, the otherwise insoluble calcite is freed 
from its bed. 
The literature on the subject of tide-pools 
is very limited. No direct references Swene 
found, but several on potholes were available. 
These structures, though lying farther inland 
than tide-pools, are somewhat similar to them in 
