chap. in. Recent Calcareous Formation. 57 



examined even a large fragment under a microscope, 

 without being able to discover the last 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 embedded and cemented. 1 One of the 

 most compact varieties, when placed in acid, was en- 

 tirely dissolved, 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 

 2'75 ; pure Carrara marble was found by Sir H. De la 

 Beche 2 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 SW., 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 SW. end of the beach, where the 

 calcareous sand is accumulating, and round which the 

 currents sweep, become gradually coated with a calcare- 

 ous incrustation, half-an-inch in thickness. It is quite 

 white, compact, with some parts slightly spathose, and 

 is firmly 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 



1 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 Geology,' book iii. ch. 17) of some eggs, containing 

 the bones of young turtles, found thus entombed. 



2 ' .Researches in Theoretical Geology,' p. 12. 



