ROCKS 49 
ferromagnesian constituent—biotite, augite, hornblende, or 
hypersthene—very often two or more of these being present. 
According to the nature of the predominant ferromagnesian 
constituent, the rock is termed Szotcte-, Hornblende-, Augite-, 
or Hypersthene-andesite. Many andesites are more or less 
markedly porphyritic—the phenocrysts being usually plagio- 
clase, and one or more of the ferromagnesian minerals. (See 
Plate III. 3.) 
Under the microscope the groundmass consists of lath-like microlites 
and crystals of plagioclase, usually fluidally arranged, with minute granules 
of ferromagnesian minerals, magnetite, etc., and with or without residual 
glass. In augite- and hypersthene-andesites the groundmass is often 
vitreous. When the whole rock is glassy it is known as Andesitic Obsidian 
or Andesitic Pitchstone. Amongst the accessory minerals in andesites are 
apatite, magnetite, sanidine, garnet, sphene, olivine, zircon, etc. Dactte 
contains quartz in addition to the essential ingredients of an andesite. 
The andesites are well represented in the Tertiary and later volcanic 
districts of Europe and America. More or less altered or decayed 
andesites are very common in the British Islands. In these rocks 
(formerly known as Porphyrites) the felspar is often kaolinised, while the 
augite and hornblende may be partially or wholly converted into chlorite, 
the hypersthene changed into bastite (a fibrous mineral with much the 
same composition as serpentine), and the magnetite into hematite. 
Gabbro is a granitoid holocrystalline aggregate of plagio- 
clase (basic soda-lime felspar) and a ferromagnesian silicate, 
the texture ranging from fine-grained to very coarsely 
crystalline. Several varieties are recognised, the distinguish- 
ing character of each being the particular ferromagnesian 
constituent that happens to be present. Thus normal 
Gabbro = plagioclase + diallage ; Morzte= plagioclase + hyper- 
sthene ; Olzvzne-gabbro and Olivine-norite contain olivine in 
addition to their essential ingredients; Mornblende-gabbro= 
plagioclase + pyroxene (diallage or hypersthene) + hornblende ; 
Mica-norite = plagioclase +hypersthene-+ biotite ; Zvoctolite= 
plagioclase+ olivine. Amongst accessory ingredients in all 
the gabbros are apatite, magnetite, ilmenite, garnet, rutile, 
spinelloids, etc. The gabbros have a mottled or speckled 
aspect, the felspar being usually bluish-white to grey, and 
the ferromagnesian mineral generally dark green. 
The gabbros are usually more or less altered, the felspar being 
frequently changed into saussurite, and the pyroxene intd smaragdite and 
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