3o6 CORALS AND CORAL LS LANDS. 



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 south-west, the rocks which are situated at the end 

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

 hard cal'Careous 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 cal- 

 careous 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 arrangement of 

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

 held in solution by the waters of the neighbouring 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 

 already explained. 



