302 IP OS t c Isia 



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 a 

 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 were 

 found, but several on potholes were available. 

 These structures, though lying farther inland 

 than tide-pools, are somewhat similar to them in 



