THE CHEMICAL COMPOSITION OE EOCKS 





bulk, than does a glassy, non-crystalline rock of the same chem- 

 ical composition. The property is therefore dependent upon 

 chemical (and consequently mineral) composition and struc- 

 ture, and as a very general rule it may be said that among the 

 igneous rocks those which contain the largest amount of silica 

 are the lightest, while those with a comparatively small amount, 

 but which are correspondingly rich in iron, lime, and magnesian 

 constituents, are proportionately heavy. 



3. THE CHEMICAL COMPOSITION OF EOCKS 



This varies naturally with their mineral composition. It is 

 customary to speak of sedimentary rocks as calcareous, sili- 

 ceous, ferruginous, or argillaceous, accordingly as lime, silica, 

 iron oxides, or clayey matter are prominent constituents. 

 Among eruptive rocks it is customary to speak of those show- 



(1) STRATIPIEI) EOCKS 



Kind 



Speoitio 

 Gravity 



Composition 



Caleareous: 

 liimestoiie 



2.6 to 2 8 



Carbonate of lime. 



Dolomite 



Siliceous : 

 Gneiss .*..... 



2.8 to 2.95 

 2.6 to 2 7 



Carbonate of lime and magnesia. 



Same as granite. 

 Mainly silica. 

 60 to 80 per cent silica. 

 Mainly silica and silicate of 

 aluminum. 



Siliceous sandstone . . 



Schist 



Clay slate (argillite) 



2.6 



2.6 to 2.8 

 2.5 



(2) ERUPTIVE ROCKS 



Kind 



Acidic group: 



Granite 



Liparite 



Obsidian 



Obsidian pumice . . 

 Intermediate group: 



Syenite 



Trachyte 



Diorite 



Andesite 



Basic group: 



Diabase 



Basalt 



Peridotite .... 



Peridotite (meteorite) 



Specipio Geavity 



} 



2.58 to 2.73 



2.53 to 2.70 

 2.26 to 2.41 

 Floats on water. 



2.73 to 2.86 

 2.70 to 2.80 

 2.70 to 2.90 



2.54 to 2.79 



2.75 to 2.95 



3.22 to 3.29 

 3.51 



Per Cent Siuca 



77.65 to 62.90 

 78.06 to 67.61 



72.20 to 54.65 



64.00 to 60.00 



62 00 to 50.00 



66.75 to 54.73 



50.00 to 48.00 



42.65 to 33.73 

 37.70 



