356 CORALS AND COMAL ISLANDS. 



of the long beach become coated by a white, thick, and very 

 hard calcareous layer. I saw portions of this remarkable 

 deposit, which had been protected by an accumulation of 

 sand. In the year 1831 it was much thicker than during any 

 other period. It would appear that the water charged with 

 calcareous matter, by the disturbance of a vast mass of calca- 

 reous particles only partially cemented together, deposits this 

 substance on the first rocks against which it impinges. But 

 the most singular circumstance is that, in the course of a 

 couple of months, this layer is either abraded or redissolved, 

 so that after that period, it entirely disappears. It is curious 

 thus to trace the origin of a periodical incrustation, on certain 

 isolated rocks, to the motion of the earth with relation to the 

 sun ; for this determines the atmospheric currents which give 

 direction to the swell of the ocean, and this again the arrange- 

 ment of the sea-beach, and this again the quantity of calcareous 

 matter held in solution by the waters of the neighboring sea." 



Mr. Darwin, speaking of a large beach of calcareous sand 

 composed of comminuted and rounded fragments of shells and 

 corals at Ascension, says, " The lower part of this, from the 

 percolation of water containing calcareous matter in solution, 

 soon becomes consolidated and is used as a building stone ; 

 but some of the layers are too hard for fracture, and when 

 struck by the hammer, ring like flint." 



The surface of hills of drift sand-rock often has small de- 

 pressions that are coated with a smooth, solid crust, as al- 

 ready explained. 



IX. FOEMATION OF DOLOMITE OR MAGNESIAN CARBONATE OF LIME. 



Analyses of the coral limestone of the elevated coral island 

 Matea, by Prof. B. Silliman,' Jr., have determined the singular 

 fact that, although the corals themselves contain very little 



