ORDER I. RHOMBOHEDBAL LIME-IIALOIDE. 91 



with in the cavities of several rocks. Slate-spar is gene- 

 rally a product of beds of granular limestone ; calcare- 

 ous tufa and rock-milk, being of a sintery formation, oc- 

 cur upon the surface and in fissures of limestone rocks, 

 and rock-milk in particular is generally a very pure car- 

 bonate of lime. Stalactitic and pisiform varieties are 

 produced by calcareous springs and other waters. The 

 original repository of Anthracolite is unknown, it having 

 as yet been found only in large boulders. A variety of 

 very similar composition, though consisting of smaller 

 individuals, forms a coat round limestone nodules in the 

 lias of Lyme in Dorsetshire ; the axis of the individuals 

 is perpendicular to the surface. The mixed or impure 

 varieties occur in particular strata, between those of com- 

 pound varieties of other species. The rhombohedral Lime- 

 haloide very often occurs in petrifactions, imbedded in 

 compact varieties of the same species. Among these we 

 notice the variety called Lumachella, which is of a dark 

 grey colour, and contains beautifully iridescent shells in a 

 high state of preservation.* 



" In various petrifactions, the disposition of the crystal- 

 line particles of the present species is very curious. Thus 

 every one of the spines of echini consists of a single indi- 

 vidual, perfectly cleavable, and the axis of which is pa- 

 rallel to the axis of the spine. But, what is still more 

 remarkable, the spines of these animals possess the same 

 property even in recent specimens of the latter, and it ap- 

 pears that the carbonate of lime crystallises as rhombohe- 

 dral Lime-haloide upon the body of those animals. The 

 process of petrifaction seems to require nothing else but 

 the replacement of the animal matter in the pores of these 

 spines by calcareous particles, which will arrange them- 

 selves according to the attraction of the crystalline particles 

 which already exist. The shell of the echini itself consists of 

 several individuals ; and the inside of each of the five am- 

 bulacra is often lined with two rows of crystals, disposed 

 like those individuals, which constitute the shell. The en- 

 crinites are likewise composed of cleavable varieties of 

 calcareous spar. The arrangement of the individuals in a 



