Note:  Do not rely on this information. It is very old.


Gabbro, a name originally applied to an Italian serpentine containing that variety of augite known as diallage; but now used for a family of plutonic rocks of basic composition and granitoid texture, consisting of anorthite or some allied basic felspar with some ferro-magnesian silicate, such as diallage, augite, or less typically hornblende or olivine. Though essentially noncrystalline, these rocks may be coarse or fine-grained: they have a specific gravity between 2.9 and 3.0; and contain from 48 to 54.6 per cent. of silica, from 10 to 29 per cent. of alumina, from 4.8 to 15.8 of iron-oxides, and from 9 to 18 per cent. of lime and magnesia. They occur in the Lizard district, Anglesey, Mull, Skye, Sutherland, Carlingford, etc., forming intrusive bosses.