258 COMPOSITION OF ANDESITES. 



under the microscope, presents the aspect of being perforated with 

 innumerable pinholes from the abundance of minute quartz grains. 



Quartz propylite forms the hills of Golconda. It is a dark-grey 

 yellow rock, with clear quartz grains about the size of peppercorns. 

 The larger felspars are more or less decomposed. The quartz contains 

 fluid inclusions with moving bubbles, and rather resembles that of 

 diorites in the matter of inclusions. No quartz is visible in the 

 ground mass. Propylites vary greatly in their percentage of silica ; 

 and in chemical composition may be instructively compared with 

 diorites, syenites, and certain slates. 



Andesites. 



Andesites are rocks which consist typically of crystals of oligoclase 

 and columnar hornblende, combined with more or less of a glassy 

 ground mass, small particles of magnetic iron,, and a few flakes of 

 mica. Augite, olivine, magnetite, and hauyine are occasionally present. 

 Andesites vary in colour from grey to dark green, and when horn- 

 blende abounds may be dark brown or black. They vary chiefly in 

 possessing or wanting an amorphous base, and, when a base exists, in 

 the relative proportion of crystals which it includes. The commonest 

 type of andesite is porphyritic, with a microcrystalline ground mass 

 which has large crystals developed in it. They are chiefly of felspar, 

 but include hornblende, mica, and quartz. Chemical analysis some- 

 times yields as much silica as occurs in a dacite. Andesites some- 

 times exhibit a fluid structure, characterised by a parallel arrangement 

 of small, slender crystals, or by the extension of such crystals in curves 

 round larger particles. 



Hornblende-andesite is commonly regarded as the volcanic equi- 



