Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.3)

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 smihng with beauty, but staring with naked