GEOLOGICAL CONCLUSIONS. 305 



between the continental sea and the existing deep oceans may 

 perhaps account for the diversity of resuUs. 



VIII. CONSOLIDATION OF CORAL ROCKS. 



All true coral-reef rocks are examples of the consolidation 

 of material mainly of coral origin — either mud, sands, frag- 

 ments, or standing corals, the last with mud or sands intermixed 

 — by (i), an under-water process ; (2), at the ordinary tempera- 

 ture; and they exemplify the mode in which all other submarine 

 limestones of organic origin have been consolidated. The 

 process appears to depend on the presence (proved. by chemical 

 analysis) of carbonic acid in the sea-waters that bathe and 

 penetrate the sands. This carbonic acid is derived from three 

 sources: from (i), the rains which wash it down from 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 mud. 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 consolidation. 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 122 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 Hme being added until all 



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