VOLCANIC OR ERUPTIVE ROCKS. 



207 



Mineral Composition and Sub-Groups. — The most striking differences 

 between the rocks of this and the other groups are found in their text- 

 ure and mode of occurrence. Mineralogically the rocks of this group 

 consist essentially of some form of feldspar, with hornblende or augite. 

 Free quartz and mica, though sometimes present, especially the former, 

 are neither necessary nor common. These also, like those of the other 

 group, may be divided into two sub-groups, acidic and basic. In the 

 one there is a predominance of orthic feldspar (sanidin) ; in the other, 

 of either hornblende or augite and clinic feldspar (plagioclase). In 

 true volcanics, as seen above, sanidin takes the place of orthoclase of 

 the Plutonics. These, however, belong to the same group (orthoclase 

 group), are equally acidic, and. therefore have the same significance in 

 lithology. The two sub-groups are, therefore, characterized by color, 

 specific gravity, and fusibility, as already explained (p. 204), and, with 

 some practice, can usually be distinguished in the field ; though in 

 many cases microscopic or chemical examination , is necessary. The 

 silica percentage of the extreme acidic type (rhyolite) is 70 to 82, and 

 specific gravity 2*3 to 2-6 ; of the extreme basic (basalt) the silica per- 

 centage is 40 to 56, and specific gravity 2*9 to 3*1. The following 

 schedule gives the most common and characteristic kinds under the 

 two sub-groups : 



VOLCANIC EOCKS. 



ACIDIC. 



BASIC. 



f Rhyolite. 

 Stony ! Liparite. 

 condition. j Trachyte. 

 L Phonolitc. 



«,„„„ ( Light-colored scoriae. 



G1 ~?y } P«mice. 

 condition. | 0bsidkn> 



Basalt. 

 Dolerite. 

 Andesite. 

 Propylite 



Black scoriae. 

 Tachylite. 



Principal Kinds. — In the acidic group the commonest and best type 

 is trachyte. This is usually a light-colored rock, with a peculiar and 

 very characteristic rough feel, due to microscopic vesicularity. It 

 consists essentially of a ground-mass of orthic feldspar (sanidin) and 

 augite, containing crystals of the former. 



Rhyolite is similar in composition to trachyte, but contains a larger 

 percentage of silica, and is very different in general appearance. It 

 consists of a fluent, vitreous ground-mass or paste, usually containing 

 crystals of sanidin, or even of quartz. When these crystals are con- 

 spicuous, so that the rock has a porphyritic appearance, it is called 

 liparite. In some cases it may have even a granitoid appearance, and 



