292 ’ THE ATLANTIC. [cHapP. Iv. 
tered into the structure of the skeletons of animals; it is there- 
fore easily soluble in water containing carbonic acid. When 
rain, which always contains a considerable quantity of carbonic 
acid derived from the atmosphere, falls upon the surface of the 
sand, it takes up a little lime in the form of bicarbonate, and 
then, as it sinks in, it loses the carbonic acid and itself evapo- 
: == 
SS = S= S= 
Fie. 76.—Aiolian Limestone Beds in process of formation, showing Stratification, and the 
Remains of a Grove of Cedars which has been overwhelmed. Elbow Bay, Bermudas. 
(From a photograph.) 
rates, and it leaves the previously dissolved carbonate of lime 
as a thin layer of cement, coating and uniting together the 
grains of sand. A crust is thus formed, and such successive 
crusts form lines of demarkation between successive layers of 
sand, and give the character of stratification and lamination 
which these wind-rocks always possess. Usually harder and 
softer layers alternate, indicating the greater or less degree in 
which the previous layer had been cemented and hardened 
before receiving the next addition of dry sand. The rocks 
