THE STRUCTURE AND CLASSIFICATION OP ROCKS. 623 



Eight non-metallic liquids and solids sulphur, phosphorus, selenium, iodine, bromine, 

 boron, carbon, and silicon. 



Three metallic bases of the alkalies potassium, sodium, and lithium. 

 Four metallic bases of the alkaline earths barium, strontium, calcium, and 

 magnesium. 



Five metallic bases of the earths aluminium, yttrium, glucinum, thorium, and 

 zirconium. 



Thirty metals, whose combinations with oxygen produce neither alkalies nor earths, and 

 which are all solid but mercury manganese, zinc, iron, tin, cadmium, arsenic, antimony, 

 copper, molybdenum, chromium, tungstenum. uranium, columbium, nickel, cobalt, cera- 

 nium, titanium, bismuth, tellurium, lead, mercury, silver, gold, platinum, palladium, 

 rhodium, osmium, iridium, vanadium, ruthenium, with eight others obscurely known. 



The most important of these elementary substances, on account of their prevalence in the 

 constitution of the globe, are oxygen, carbon, sulphur, aluminium, silicon, potassium, 

 sodium, calcium, magnesium, and iron, a specific description of which may be found in 

 any chemical treatise. The preceding ingredients, either singly or combined, compose 

 the simple minerals, differing from each other in shape, colour, lustre, and hardness ; and 

 all rocks are either mineral aggregations, or formations of only one mineral. The lead 

 ing mineral substances of which rocks are composed are the following : 



Quartz, or siliceous earth. This is one of the hardest and most abundant substances in 

 nature. It forms independent mountainous masses ; occurs in veins intersecting moun 

 tains ; is a constituent part of flint, chalcedony, rock-crystal, sandstone, and granite ; and 

 is found in some vegetable productions, as the stalks of reeds, and the outer coating of the 

 bamboo, which will strike fire with steel. The white crystals of granite, and the white 

 grains of sandstone, are of quartz. 



Felspar, a compound of siliceous and clay earths, with a portion of potassia, lime, and 

 oxide of iron, is also an abundant mineral, but less hard than quartz. The soft gray 

 crystals of granite, which can easily be scratched, are of felspar. It is sometimes white, 

 cream coloured, or red, passing through various shades, according to the quantity of the 

 oxide of iron. Containing a large proportion of aluminum or clay earth, felspar is 

 employed in the manufacture of pottery ware, Cornwall furnishing the principal part of 

 that used in England. The Pentland Hills are almost entirely composed of it ; but it is 

 there too much tinged with the metallic oxide to be serviceable in the arts. 



Mica, a term derived from the Latin micans, glittering, is a compound of alumina, 

 silica, magnesia, and oxide of iron, and is so called from its lustrous appearance. The 

 transparent portions of granite are of mica, and it occurs in many sandstones, giving them 

 a silvery aspect. 



Talc, a substance closely resembling mica, is distinguishable from it by being softer 

 and more brittle and inflexible. 



Chlorite, nearly allied to talc, differs from it in coloui*, deriving its name from the 

 Greek chloros, green. In the composition of many of the Alpine granite rocks, talc and 

 chlorite take the place of mica. 



Hornblende, a combination of silica, alumina, magnesia, lime, oxide of iron, and man 

 ganese, is of a deep green colour, and sometimes black. It is a prevalent ingredient in 

 many igneous masses, occurring in beds, veins, and granular pieces in compound rocks, 

 and forming entire mountains, the trap rocks consisting of it. Of a similar composition 

 are the greenish minerals augite and actynolite. Serpentine contains the same ingredients 

 as hornblende, but in different proportions, having more magnesia and less iron. It 

 derives its name from its spotted colours, resembling those of the serpent's skin. 



Carbonate of lime, the constituents of which are lime and carbonic acid, enters largely 



