Volcanic Islands visited during the Voyage of the Beagle. 349 



and volcanic rock can be distinctly seen to be enveloped in a husk of pel? 

 lucid carbonate of lime. Extremely few perfect shells are imbedded in 

 these agglutinated masses; and I have examined even a large fragment 

 under a microscope, without being able to discover the least vestige of striae 

 or other marks of external form : this shows how long each particle must 

 have been rolled about, before its turn came to be imbedded and cemented*. 

 One of the most compact varieties, when placed in acid, was entirely dis- 

 solved, with the exception of some flocculent animal matter; its specific 

 gravity was 2"63. The specific gravity of ordinary limestone varies from 

 2-6 to 275; pure Carrara marble was found by Sir H. De la Bechef to be 

 2'7. It is remarkable that these rocks of Ascension, formed close to the 

 surface, should be nearly as compact as marble, which has undergone the 

 action of heat and pressure in the plutonic regions. 



"The great accumulation of loose calcareous particles, lying on the 

 beach near the Settlement, commences in the month of October, moving 

 towards the S.W., which, as I was informed by Lieut. Evans, is caused by 

 a change in the prevailing direction of the currents. At this period the 

 tidal rocks, at the S.W. end of the beach, where the calcareous sand is ac- 

 cumulating, and round which the currents sweep, become gradually coated 

 with a calcareous incrustation, half an inch in thickness. It is quite white, 

 compact, with some parts slightly spathose, and is firndy attached to the 

 rock. After a short time it gradually disappears, being either redissolved, 

 when the water is less charged with lime, or more probably is mechanically 

 abraded. Lieut. Evans has observed these facts during the six years he 

 has resided at Ascension. The incrustation varies in thickness in different 

 years: in 1831 it was unusually thick. When I was there in July, there 

 was no remnant of the incrustation ; but on a point of basalt, fn)m which 

 the quarrymen had lately removed a mass of the calcareous freestone, the 

 incrustation was perfectly preserved. Considering the position of the tidal 

 rocks, and the period at which they become coated, there can be no doubt 

 that the movement and disturbance of the vast accumulation of calcareous 

 particles, many of them being partially agglutinated together, causes the 

 waves of the sea to be so highly charged with carbonate of lime, that they 

 deposit it on the first objects against which they impinge. I have been in- 

 formed by Lieut. Holland, R.N., that this incrustation is formed on many 

 parts of the coast, on most of which, I believe, there are likewise great 

 masses of comminuted shells." 



This is succeeded, as intimated above, by a minute description il- 

 lustrated by a figure, of a frondescent incrustation of calcareous and 

 animal matter, coating throughout the year the tidal volcanic rocks 

 at Ascension, that project from the beaches composed of broken 

 shells ; the origin of this he shows, is due to the solution and sub- 

 sequent deposition of matter derived from the rounded particles of 

 corals and shells, which form the nuclei of the fronds or discs of 

 which the incrustation is composed, mingled with minute lamellae of 

 selenite derived from the sea- water ; and it is an interesting circum- 

 stance, he remarks, thus to find the waves of the ocean sufficiently 

 charged with sulphate of lime, to deposit it on the rocks, against 

 which they dash every tide. When the incrustation now described 



* " The eggs of the turtle being buried by the parent, sometimes become 

 enclosed in the solid rock. Mr. Lyell has given a figure (Principles of Geo- 

 logy, book iii. ch. 17) of some eggs, containing the bones of young turtles, 

 found thus entombed." 



t " Researches in Theoretical Geology," p. 12. 



