chap. in. Calcareous Incrustation. 61 



From this source it derives its animal matter, which 

 is evidently the colouring principle. The nature of 

 the deposit, in its incipient stage, can often be well 

 seen upon a fragment of white shell, when jammed be- 

 tween two of the fronds ; it then appears exactly like 

 the thinnest wash of a pale gray varnish. Its darkness 

 varies a little, but the jet blackness of some of the 

 fronds and of the botryoidal masses seems due to the 

 translucency of the successive gray layers. There is, 

 however, this singular circumstance, that when de- 

 posited on the under side of ledges of rock or in fissures, 

 it appears always to be of a pale, pearly gray colour, 

 even when of considerable thickness : hence one is led 

 to suppose, that an abundance of light is necessary to 

 the development of the dark colour, in the same 

 manner as seems to be the case with the upper and ex- 

 posed surfaces of the shells of living mollusca, which 

 are always dark, compared with their under surfaces 

 and with the parts habitually covered by the mantle of 

 the animal. In this circumstance, — in the immediate 

 loss of colour and in the odour emitted under the blow- 

 pipe, — in the degree of hardness and translucency of 

 the edges, — and in the beautiful polish of the surface, 1 

 rivalling when in a fresh state that of the finest Oliva, 

 there is a striking analogy between this inorganic in- 



cleer,"' vol. ii. p. 319) beds of gypsum and salt, as much as two feet 

 in thickness, left by the evaporation of the spray on the rocks on 

 the .wind ward coast. Beautiful stalactites of selenite, resembling 

 in form thos-e of carbonate of lime, are formed near these beds. 

 Amorphous masses of gypsum, also, occur in caverns in the interior 

 of the island ; and at Cross Hill (an old crater) I saw a considerable 

 quantity of salt oozing from a pile of scoria?. In these latter cases, 

 the salt and gypsum appear to be volcanic products. 



1 From the fact described in my ' Journal of Researches ' (p. 12), 

 of a coating of oxide of iron, deposited by a streamlet on the rocks 

 in its bed (like a nearly similar coating at the great cataracts of the 

 Orinooco and Nile), becoming finely polished where the surf acts, I 

 presume that the surf in this instance, also, is the polishing agent. 



