s6 Ascension. 



PAKT L 



which are lined, both by black, metallic, and by other 

 white and crystalline specks, the nature of which I was 

 unable to ascertain. Some of the larger concretions 

 consist of a mere spherical shell, filled with slightly 

 consolidated ashes. The concretions contain a small 

 proportion of carbonate of lime : a fragment placed 

 under the blowpipe decrepitates, then whitens and 

 fuses into a blebby enamel, but does not become caustic. 

 The surrounding ashes do not contain any carbonate of 

 lime ; hence the concretions have probably been formed, 

 as is so often the case, bv the agereo-ation of this sub- 

 stance. I have not met with any account of similar 

 concretions ; and considering their great toughness and 

 compactness, their occurrence in a bed, which probably 

 has been subjected only to atmospheric moisture, is 

 remarkable. 



Formation of calcareous rocks on the sea-coast. — 

 On several of the sea-beaches, there are immense accu- 

 mulations of small, well-rounded particles of shells and 

 corals, of white, yellowish, and pink colours, inter- 

 spersed with a few volcanic particles. At the depth of 

 a few feet, these are found cemented together into 

 stone, of which the softer varieties are used for build- 

 ing; there are other varieties, both coarse and fine- 

 grained, too hard for this purpose : and I saw one 

 mass divided into even layers half-an-inch in thickness, 

 which were so compact that when struck with a hammer 

 they rang like flint. It is believed by the inhabitants, 

 that the particles become united in the course of a 

 sino-le vear. The union is effected bv calcareous 

 matter ; and in the most compact varieties, each 

 rounded particle of shell and volcanic rock can be dis- 

 tinctly seen to be enveloped in a husk of pellucid 

 carbonate of lime. Extremely few perfect shells are 

 embedded in these agglutinated masses ; and I have 



