54 THE EARTH. 



gneiss, which consists of the same ingredients as granite in 

 a slightly altered form. Granite will often pass into gneiss 

 gradually, and this again into those slaty formations which 

 rest upon it and come next in the series. These consist of 

 mica-slate, hornblend-slate, and chlorite-slate or schist, a 



name given to all those rocks of a slaty structure which 

 have a cleavage or capability of being cleft into thin laminae 

 or slices (hence their applicability to the purposes of building, 

 forming excellent roofing materials, &c.). 



All these rocks have received the name of "primary," 

 they have no appearance of being stratified or deposited in 

 layers, which appearance seems to be in all cases the result 

 of having at some time been suspended in water and thence 

 deposited gradually as mud, sand, &c., time, pressure, and 

 heat having afterwards altered their consistence. All these 

 consist of the earths described as composing granite, but 

 combined in different proportions. Above these rocks there 

 is a formation of a totally different composition, namely 

 crystalline limestone, commonly known as marble; this 

 consists wholly of carbonate of lime or lime in union with 

 carbonic acid, and its crystalline state appears to have been 

 produced by long-continued heat and pressure. This lime- 

 stone is by many ranked amongst the primary rocks, although 

 it differs so entirely in its composition which exactly 

 resembles a formation presently to be described (the chalk) ; 

 however, if this be classed with the primary rocks, then it 

 may be fairly said that all above have resulted from their 

 disintegration and reunion in different forms, by the action 

 of water and vital forces of different kinds. The state of 



