LIMESTONE. GYPSUM. SLATE. 37 



The intimate connection between hornblende and serpentine is 

 now completely established ; for hornblende is observed to be chang- 

 ed into serpentine, by contact with limestone in various situations. 

 Serpentine, sometimes, occurs crystallized, and has received the 

 name of diallage. 



Limestone [Carbonate of Lime), however various in external ap- 

 pearance it may be, is, if pure, essentially composed of 57 parts of 

 lime, and 43 carbonic acid ; but in some rocks the limestone is inter- 

 mixed with magnesia, alumine, silex, or iron. The specific gravity 

 of limestone varies from 2*50 to 2-80. All limestones may be scra- 

 ped with a knife. They are infusible ; but, when impure by an in- 

 termixture with a portion of other earths, they vitrify in burning. All 

 limestones effervesce, when a drop of strong acid is applied on the 

 surface ; and they dissolve, entirely, in nitric or muriatic acid. The 

 specific gravity, hardness, and effervescence with acids, taken col- 

 lectively, distinguish limestone from all other minerals. 



Crystallized Carbonate of Lime [Calcareous Spar) occurs, crys- 

 tallized, in a great variety of forms ; the crystals break, easily, with 

 the stroke of a hammer, and the fragments are always rhomboidal. 



Vast mountains and extensive strata of limestone cover a large 

 portion of many countries. The varieties of limestone will be des- 

 cribed, as the rocks occur, in the primary or secondary series. The 

 different appearance of statuary marble and chalk is well known to 

 every one. They are only different modifications of limestone, and 

 are chemically the same. Magnesian limestone, sometimes called 

 Dolomite, possesses most of the physical characters of common 

 limestone, but contains various proportions of magnesia. 



Gypsum, or Sulphate of Lime, is far less abundant than carbonate 

 of lime ; but it forms, in some situations, beds of considerable thick- 

 ness and extent. Gypsum is, generally, of a color inclining to white, 

 and is sometimes snow-white. Common Gypsum has a laminated 

 or granular structure, and is, sometimes, compact. Jt is much softer 

 than common limestone and may be scratched with the nail ; it does 

 not effervesce with acids. Crystallized gypsum has the properties of 

 common gypsum ; it is frequently called selenite. The constituent 

 parts of gypsum are lime 32*7, sulphuric acid 46*3, and water 21. 

 A variety of gypsum which has no water in its composition, and 

 hence called anhydrous, occurs in beds in the Savoy Alps ; it is there 

 combined with siliceous earth. It is much harder than common 

 gypsum, and even than common limestone. The specific gravity of 

 common gypsum varies from 2*16 to 2-28; that of anhydrous gyp- 

 sum is from 2*8° to 2*90. Gypsum, under the name of plaster 

 stone, is a mineral generally known. 



Slate, improperly called by some geologists clay-slate, and by the 

 old geologists argillaceous schistus, is well known, — at least the com- 

 mon variety used as roofing slate, which may be regarded as the pu- 

 rest form of this mineral. 



