Parr il. § vi] CRYSTALLINE ROCKS—MASSIVE. 147 
the case, that, as above remarked, rocks now known io be of Tertiary 
date, have been described as melaphyres, while others of Lower 
Carboniferous age have been unhesitatingly referred to as basalts. 
Augite-Andesite is the name given to certain dark eruptive 
rocks of Tertiary and post-Tertiary date, which consist of a triclinic 
felspar (oligoclase, or some species rather richer in silica than 
labradorite) and augite, with sometimes sanidine, hornblende, biotite, 
magnetite, or apatite, and in some varieties quartz. The ground- 
mass is resolvable under the microscope, sometimes into a glassy 
sometimes into a more or less fully devitrified base. The quartz- 
bearing varieties contain from 63 to 67 per cent. of silica, and 
in this respect, as well as in the failure of olivine, are distinguished 
from the basalts. The average composition of the quartzless varieties 
may be thus given: Silica, 57°15; alumina, 16°10; protoxide of iron, 
13-0; lime, 5°75; magnesia, 2°21; potash, 1°81; soda, 3°88; mean 
specific gravity, 2°75—2°85. : 
Augite-Andesite occurs in dykes, lava streams, plateaux, sheets 
and neck-like bosses in regions of extinct and active volcanoes, as in 
Transylvania and Hungary, Santorin, Iceland, Teneriffe, the Western 
Territories of North America, the Andes, New Zealand, &c. 
Basalt-Rocks.’— Under this title is embraced an important and 
widespread series of volcanic rocks, which consist essentially of some 

Fic. 24.—Microscoric STRUCTURE OF BASALT. 
The large shaded Crystals are Olivine considerably serpentinized; the numerous small 
white Prisms are Plagioclase. A few Augite prisms occur which, to the right 
of the centre cf the drawing, are aggregated into a large compound crystal. The 
black specks are Magnetite. 
triclinic felspar, augite, olivine, magnetite or titaniferous iron, 
frequently with apatite, sometimes with sanidine or nepheline. 
1 Ante pp. 109, 145. See also Trans. Roy. Soc. Edin., vol. xxix. p. 499. 
2 On basalt rocks see Zirkel’s Basaltgestcine, 1870. Boricky’s “ Petrographische 
Studien an den Basaltgesteinen Bohmens,” in Archiv. fiir Naturwiss. Landesdurch- 
forschung von Béhmen, ii. 1873. Allport, Q. J. Geol. Soe. xxx. p. 529. Geikie, Trans. 
Roy. Soc. Edin. xxix. Mohl, Nov. Act. Acad. Leop. Carol. xxxvi. (1873) p. 74; Neues 
Jahrb. 1873, pp. 449, 824. 9 
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