6o Ascension. 



PAKT I. 



may often be traced, either to a minute particle of shell 



fixed in a crevice of the rock, or to several cemented 

 together ; these first become deeply corroded, by the 

 dissolving power of the waves, into sharp ridges, and 

 then are coated with successive layers of the glossy, 

 gray, calcareous incrustation. The inequalities of the 

 primary support affect the outline of every successive 

 layer, in the same manner as may often be seen in 

 bezoar-stones. when an object like a nail forms the 

 centre of ap-greQ'ation. The crenulated edges, how- 

 ever, of the frond appear to be due to the corroding 

 power of the surf on its own deposit, alternating with 

 fresh depositions. On some smooth basaltic rocks on 

 the coast of St. Jago, I found an exceedingly thin layer 

 of brown calcareous matter, which under a lens pre- 

 sented a miniature likeness of the crenulated and 

 polished fronds of Ascension ; in this case a basis was 

 not afforded by any projecting extraueous particles. 

 Although the incrustation at Ascension is persistent 

 throughout the year ; yet from the abraded appearance 

 of some parts, and from the fresh appearance of other 

 parts, the whole seems to undergo a round of decay 

 and renovation, due probably to changes in the form 

 of the shifting beach, and consequently in the action 

 of the breakers : hence probably it is, that the incrusta- 

 tion never acquires a great thickness. Considering the 

 position of the incrusted rocks in the midst of the cal- 

 careous beach, together with its composition, I think 

 there can be no doubt that its origin is due to the dis- 

 solution and subsequent deposition of the matter com- 

 posing the rounded particles of shells and corals. 1 



1 The selenite, as I have remarked, is extraneous, and must have 

 been derived from the sea -water. It is an interesting circumstance 

 thus to find the waves of the ocean, sufficiently charged with sul- 

 phate of lime, to deposit it on the rocks, against which they dash 

 every tide. Dr. Webster has described (' Voyage of the w Chanti- 



