chap. in. Calcareous Incrustation. 59 



posed, are generally so closely crowded together as to 

 touch. These fronds have their sinuous edges finely 

 crenulated, and they project over their pedestals or 

 supports ; their upper surfaces are either slightly con- 

 cave, or slightly convex ; they are highly polished, and 

 of a dark gray or jet black colour ; their form is irre- 

 gular, generally circular, and from the tenth of an inch 

 to one inch and a-half in diameter ; their thickness, or 

 amount of their projection from the rock on which they 

 stand, varies much, about a quarter of an inch being per- 

 haps most usual. The fronds occasionally become more 

 and more convex, until they pass into botryoidal masses 

 with their summits fissured ; when in this state, they are 

 glossy and of an intense black, so as to resemble some 

 fused metallic substance. I have shown the incrustation, 

 both in this latter and in its ordinary state to several 

 geologists, but not one could conjecture its origin, except 

 that perhaps it was of volcanic nature ! 



The substance forming the fronds has a very com- 

 pact and often almost crystalline fracture ; the edges 

 being translucent, and hard enough easily to scratch 

 calcareous spar. Under the blowpipe it immediately be- 

 comes white, and emits a strong animal odour, like that 

 from fresh shells. It is chiefly composed of carbonate 

 of lime ; when placed in muriatic acid it froths much, 

 leaving a residue of sulphate of lime, and of an oxide of 

 iron, together with a black powder, which is not soluble 

 in heated acids. This latter substance seems to be car- 

 bonaceous, and is evidently the colouring matter. The 

 sulphate of lime is extraneous, and occurs in distinct, 

 excessively minute, lamellar plates, studded on the sur- 

 faces of the fronds, and embedded between the fine 

 layers of which they are composed ; when a fragment is 

 heated in the blowpipe, these lamellas are immediately 



rendered visible. The original outline of the fronds 



4 



