624 GEOLOGY. 



into the composition of animal bodies, forming the solid parts of the bones and teeth, and 

 the shells of many molluscs. It enters also into the composition of vegetables, is diffused 

 in the ocean in the form of muriate of lime, forms marble, chalk, and limestone, of various 

 degrees of hardness, and is supposed to constitute one seventh of the crust of the globe. 

 It exists in great purity in calcareous spar, and in the Carrara marble used in statuary ; 

 but limestone often occurs intermixed with other ingredients, as magnesia, alumina, silica, 

 or iron. 



Iron, in some combination, as an oxide, sulphuret, or carburet, occurs abundantly, and 

 is supposed to constitute at least three per cent, of all known rocks. Rust is an oxide of 

 iron ; pyrites, or the small yellow cubes found in roofing slate, a sulphuret ; and graphite, 

 a carburet, also known under the names of plumbago and black lead, is found in great 

 purity in Borrowdale in Cumberland. 



These are the simple minerals which form the great mass of rocks, though others com- 

 pose large independent beds, or occur extensively in connection with those enumerated, 

 modifying the character of their respective structures ; as sulphate of lime, or gypsum, 

 the plaster-stone of commerce ; chloride of sodium, or common salt, found in sea-water, 

 and in masses constituting rock-salt ; bitumen, found liquid in petroleum or rock-oil, solid 

 in asphalte, and mixed in common coal ; garnet, schorl, and steatite. Rocks are either 

 simple, formed by a single mineral, or compound, several uniting in their composition, in 

 general from two to four. Thus quartz, limestone, and common salt exist by themselves 

 in large masses ; while granite is a compound of the crystals of quartz, felspar, and mica, j 

 Some rocks appear simple, which are in reality compounds, owing to the different 

 materials of which they consist having been so much ground down, previous to conso- 

 lidation, as to make the formation appear homogeneous, as in several varieties of shale 

 and slate. Among the compound rocks, the granitic have their grains or crystals 

 united together without a cement ; in the porphyritic, distinct crystalline masses, of a 

 different composition from the base of the rock, are imbedded in it, as in a kind of paste ; 

 in the amygdaloidal, there are round or kernel-shaped cavities, filled with mineral 

 matter, distinct from that which forms the basi*. Angular fragments of rock, of the 

 same or different kinds, cemented together by iron or carbonate of lime, infiltrated 

 through the mass in a state of solution, constitute a breccia ; and large fragments of 

 stone, whether angular or rounded, imbedded in strata of hardened clay or sand, form a 

 conglomerate. 



From the composition of rocks, which includes their chemical and mineral character, 

 we proceed to notice their mechanical structure, or the internal and external appearances 

 which they present. 



The Internal Structure of rocks refers to the manner in which the constituents of each 

 particular species are arranged, the leading varieties of which are the granular, the 

 fibrous, the porous, and the laminar. The texture of the granular exhibits distinct grains 

 of different sizes, as granite, which frequently assume a regular and well-defined 

 crystalline form, the several crystals being confusedly mixed and compacted together, at 

 all points interfering with each other, exhibiting the appearance of a simultaneous 

 formation. The fibrous texture is a compost of long and minute fibres, as in asbestos, 

 which is called acicular when the fibres have a distinct needle-shaped appearance. 

 Pumice-stone is an example of the porous texture, being penetrated by pores, which is 

 styled cellular, or vesicular, when the pores swell into rounded cavities resembling cells 

 or vesicles. The laminar texture also called slaty, fissile, and schistose is an arrange- 

 ment of the substance of rocks in thin plates or divisions. It appears in gneiss, felspar, 

 and mica-slate. Mica is readily split into thin, flexible, and transparent plates, which 

 before the general introduction of glass were frequently used to form window-panes, and 



