72 THE VOYAGE OF H.M.S. CHALLENGER. 



the Residence in the beginning of October, the sand being driven towards the south- 

 west. According to Lieutenant Evans, this is accounted for by a change in the pre- 

 vailing direction of the currents. During this period the rocks exposed to the tide on 

 the south-west are gradually covered by a calcareous incrustation, the thickness of 

 which may attain half an inch. This coating adheres strongly to the rock, is white 

 in colour, and at some points laminated, but after the lapse of a certain time it dis- 

 appears ; perhaps it is re-dissolved by the sea water, perhaps worn away by the waves. 

 Lieutenant Evans, who communicated these observations to Darwin, had had oppor- 

 tunities of studying the phenomena during six years which he spent at Ascension. 

 The thickness of the layer varied from year to year; in 1831 it was exceptionally 

 great. When Darwin landed in June 1839, he could only see it at one point above a 

 basaltic rock from which the quarrymen had raised a block of limestone. On taking 

 into account the position of the rocks exposed to the tide, and the period at which 

 they are covered with the calcareous coating, one comes to the conclusion that 

 the sea water, continuously in contact with the particles of broken shells on the 

 beach, takes up an excess of calcium carbonate, and then on evaporation deposits 

 it upon the rocks over which the waves wash. According to information given to 

 Darwin by Lieutenant Holland, this incrustation is found on the rocks of the coast 

 in several parts of the island. 1 The formation of this deposit must be explained by 

 the solvent action of sea water on the shelly formations of the shore, and the rapid 

 evaporation of the water. 



The specimens of these oolitic rocks which we have examined come from the west 

 coast, and vary greatly in coherence. Some are scarcely compact, the fragments of 

 shells and minerals being simply brought together without the aid of calcareous 

 cement ; others are massive, very coherent, and hard, showing a compact ground-mass 

 in which the naked eye can detect the pink or white organic particles mixed with 

 black volcanic grains. 



Microscopic observation shows that the fragments cemented together by calcareous 

 matter are all perfectly rounded, the elliptical form sometimes prevailing. They are 

 composed of the remains of shells and other organic debris, and are distinguished from 



1 Besides this deposit and the rocks formed of shell fragments, Darwin describes a calcareous incrustation 

 presenting a special structure. It also covers volcanic rocks exposed to the tide. We have found nothing amongst 

 the specimens to correspond to the description and figure he gives in his book on Volcanic Islands, p. 51. We 

 refer to the passage where the author enters into very precise details on the subject of this layer, the form of 

 which closely resembled an organic structure. He considers it due to the same cause as the cemented limestone and 

 incrustations of the coast. In the analysis he made of the concrelionary inciustation of Ascension, calcium sulphate 

 was found ; this might come from evaporated sea water. He adverts to Dr. Webster's description (Voyage of the 

 " Chanticleer," vol. ii. p. 319) of beds of gypsum and salt, 2 feet thick, on rocks exposed to the prevailing wind. 

 Fine gypsum stalactites, resembling those of carbonate of lime, may be seen there. In the caves of the centre of the 

 island amorphous masses of gypsum are found, and in an old crater on Cross Hill the salt appears traversing the 

 scorise. In this case Darwin considers the sea salt and gypsum as of volcanic origin (see Darwin, Geol. Obs., p. 53, 

 footnote). 



