90 PHYSIOGRAPHY. CLASS II. 



vity. Calcareous tufa, a recent deposit formed on the sur- 

 face of the earth, is often cleavable, and then possesses all 

 the properties of calcareous spar. Slate-spar is produced by 

 a la:nellar composition in massive varieties in the direction 

 of the face of It os, contained in thin parallel layers. 

 The face of composition, like that of R co in crystallised 

 individuals, often possesses pearly lustre. The single laminae 

 of slate-spar are cleavable into rhombohedrons, like every 

 other simple variety of the species. There is no transition 

 from slate-spar into Aphrite. Sitnncstonc, Anthracolite^ 

 Marl, Duttenstcin, and bituminous Marlslate, are impure and 

 mixed varieties, partly of calcareous spar, partly of com- 

 pact limestone. 



The pure varieties of rhombohedral Lime-haloide con- 

 sist, according to the analyses of several of the first che- 

 mists, of 



Lime 56*0... 57-0. 



Carbonic Acid 43-0... 44-0. 



Their chemical formula is Ca C 2 , which requires 56-39 of 

 lime, and 43-61 of carbonic acid. Very often the varieties 

 contain a small proportion of oxide of iron, silica, magnesia 

 alumina, carbon or bitumen. Rhombohedral Lime-haloide, 

 if pure, is entirely soluble in nitric acid, during which a 

 brisk effervescence takes place. In common fire it is 

 infusible, but loses its carbonic acid, and becomes burnt, or 

 quick lime. 



3. Rhombohedral Lime-haloide rarely enters into the 

 composition of rocks. In most cases, the more consider- 

 able masses of it form particular beds in other rocks, or 

 constitute rocks themselves : the latter consist chiefly, 

 though not exclusively, of compact limestone ; the former 

 of granular limestone. The simple varieties occur in drusy 

 cavities, more frequently in veins than in beds, accom- 

 panied with the varieties of different species. There are 

 also compound varieties found in these repositories; co- 

 lumnar compositions have been observed to form veins 

 by themselves, and a great number of varieties are met 



