GEOLOGICAL CONCLUSIONS. 355 



the atmosphere ; (2) the respiration of all the animal life in 

 the waters, even down to the simplest and minutest ; and (3) 

 the decomposition of all vegetable or animal debris in the 

 waters or diffused through the sand or muds. This gas is set 

 free, therefore, just where it is needed for the work, and is 

 always ready to perform its part in the process of consolida- 

 tion. It enables the water to take up carbonate of lime from 

 the grains of the mass to be solidified, or from outside sources ; 

 and then the deposition of the same among the grains through 

 their attractions produces the cementation. 



The beach and drift sand-rocks or oolites are different 

 from the reef-rock in being superficial deposits. The carbonic 

 acid of the waters performs the same part as in the latter ; but 

 with these, there is alternate wetting and drying during the 

 ebb and flow of the tides and the succession of gales and quiet 

 winds. By this means, the grains become incrusted, and every 

 new wetting and drying adds a new layer to the surface of each ; 

 and thus the oolitic structure is produced. Facts are men- 

 tioned on page 153 of pebbles of volcanic or basaltic rocks, 

 lying loose on a seashore, becoming incrusted in this way with 

 a milky layer ; and of basaltic conglomerates being made by 

 the same means, the carbonate of lime being. added until all 

 the intervals between the stones were filled up and the whole 

 made solid ; and of an amygdaloidal volcanic rock on a coast 

 having derived its little calcareous kernels or amygdules from 

 the same source. The following additional facts are cited 

 from Mr. Darwin's Journal (p. 588) : 



" Lieutenant Evans informs me that during the six years 

 he has resided on this island (Ascension) he has always ob- 

 served that in the months of October and November, when 

 the sand [of a calcareous beach] commences travelling to- 

 ward the southwest, the rocks which are situated at the end 



