588 



ASCENSION. 



July, 1836. 



as a building stone ; but some of the layers are too hard for 

 freestone, and when struck by the hammer ring like flint. 



The main line of beach is directed N.E. and S.W. ; 

 Lieutenant Evans informs me, that during the six months 

 included between the 1st of April and the 1st of October, 

 the sand accumulates towards the N.E. extremity, and 

 during the other six it travels back again towards the S.W. 

 end. This periodical movement is due to a change in the 

 direction of the swell, which is influenced by the general 

 direction of the trade-wind, during the two periods of the 

 year. Lieutenant Evans also informs me that during the 

 six years he has resided on this island, he has always 

 observed, that in the months of October and November, 

 when the sand commences travelling towards the S.W., the 

 rocks which are situated at that end 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 calcareous 

 particles, only partially cemented together, deposits this sub- 

 stance 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 the direction to the swell of the ocean, 

 and this acts on the arrangement of the sea-beach, and this 

 again on the quantity of calcareous matter held in solution 

 by the waters of the neighbouring sea. 



One of my excursions took me towards the S.W. ex- 

 tremity of the island. The day was clear and hot, and I saw 

 the island, not smiling with beauty, but staring with naked 



