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190 



GEOLOGY. 



r .^t. VI. 



for Instance, where a line of cliffs, too irregular to have 

 been formed by elevation alone, plunges precipitously into 

 a sea so profoundly deep that it cannot be supposed that 



the now deeply submerged portions of the cliff have been 



simply worn away by the currents. The direct evidence 

 of subsidence, if not witnessed by man, is almost confined 

 to the presence of stumps of trees, peat-beds, and ruins 

 of ancient buildings, partly submerged on tidal beaches. 

 Ancient buildings may sometimes afford such evidence 

 in imlikely situations : it has been asserted, that in one 

 of the volcanic islands in the Caroline archipelago there 

 ruins with the steps covered by the sea. A^-ain, at 

 Terceira, at the A.s^ores, there is an old church or monas- 

 tery said to be similarly circumstanced. 



Coral Beefs* — The most important point with re- 

 spect to coral reefs, which can be investigated, is, the 





depth at which the bottom of the 



/, 



ceases to be covered with a continuous bed of living 

 corals. This can be ascertained by repeated soundings 



with a 



heavy and vei-y broad bell-shaped lead, armed 

 with tallow, which Mall break off minute portions of the 

 corals or take an exact impression of thu... . it can thus 

 also instantly be seen how soon the bottom becomes covered 

 with sand. This limit of depth ought to be ascertained 

 in diirercnt seas, under different latitudes, and under dif- 

 ferent exposures. For collecting specimens of the corals, 

 it is to be feared that the dredge „ould become entangled, 

 but chains and hooks may be lowered for this purpose. 

 There is reason to suspect that different species of corals 



* The only -n'ork specially writion on this suln'ect is ' The Structure 

 and Dist!'--fiou of Corai Keefs; by Mr. Darwin. 



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