NATURE AND TYPES OF BASALT. 



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Basalt. 



Basalt, anamesite, and dolerite, are typical basaltic rocks, which 

 vary in texture from the compact condition of black biscuit china 

 seen in basalt, to a finely granular state in dolerite. These lavas 

 have a dark colour on the newly-fractured surface, varying through 

 shades of greyish brown, blue, and greenish black ; but where the 

 external surface is weathered, the rock is commonly a pale drab, 

 though the tint varies with chemical and mineral composition and 

 texture. Basaltic rocks have a high specific gravity and basic com- 

 position. Their silica rarely sinks below 40 per cent. ; a lower 

 percentage of silica is usually associated with large percentages of 

 iron, and sometimes of lime. The silica rarely exceeds 56 per cent. 

 The alumina has no necessary relation to the silica, though the average 

 amount ranges between n per cent, and 28 per cent. The lime, 

 magnesia, potash, and soda all vary in amount, and on this variation 

 depends the mineral composition of the rock. Basalt abounds in 

 labradorite and augite, generally contains magnetite and olivine, and 

 sometimes may have a little quartz and sanidine. 



Zirkel's Classification of Basalt. Zirkel distinguished several 

 varieties of basalt according to the texture of the rock, which is 

 sometimes even-grained, and free from ground mass and porphyritic 

 crystals ; or it may be very fine-grained with porphyritic crystals, and 

 only a trace of glass ; or there may be a homogeneous glassy or half- 

 glassy ground mass half-filled with crystals ; or there may be large 

 crystals in an ill-defined amorphous mass. 



But besides these varieties of texture due to conditions of cooling, 

 almost every locality affords varieties distinguished by mineral com- 

 position ; and sometimes this variation is so marked that the felspar 



