ie: ied GEOGNOSY. [Boox IT. 
aggregate of a triclinic felspar (sometimes, however, saussurite) 
and diallage or smaragdite. ‘The felspar (usually taken to be 
labradorite) occurs in distinct crystals or crystalline aggregates of 
grey, white or violet tint, and under the microscope is sometimes found 
to be crowded with crystallites. The saussurite is likewise light- 
coloured, while the diallage is distinguishable by its dirty-green or 
brown tint, the metalloidal or pearly lustre on its cleavage planes, 
and the frequent presence of layers of microscopic dark brown or 
black lamelle. Some varieties contain abundant olivine. Average 
composition—silica, 49; alumina, 15; lime, 9°5; magnesia, 9:7; 
oxides of iron and manganese, 11°5; potash, 0°3; soda, 25; loss by 
ignition, 2°5; specific gravity, 2°85—3:10. 
Gabbro occurs (1) in association with granite, gneiss, and other 
crystalline rocks as large irregular bosses (Saxony, Silesia, the 
Harz, &c.), and (2) in large sheets and bosses associated with 
volcanic eruptive rocks. In the latter case it occurs in Skye and 
Mull connected with Miocene volcanic outflows." 
Hypersthenite, allied to gabbro, is a granular granitoid agere- 
gate of labradorite and hypersthene, found in beds, bosses, and veins, 
in Norway, Greenland, and Labrador. 
(2) Nepheline Rocks. 
Under this name is grouped a series of distinctly crystalline 
and also compact dark rocks composed of nepheline, augite, and 
magnetite, often with olivine, sometimes with a little triclinic felspar. 
They are thus distinguished by the fact that in them the part taken 
by felspar in the rocks already enumerated is supplied by nepheline. 
They are usually divided into nepheline-dolerite, a crystalline 
granular ageregate closely resembling in general character true 
dolerite; and nepheline-basalt, a black, heavy compact rock not to 
be outwardly distinguished from ordinary felspar-basalt. They are 
volcanic masses of late Tertiary age, but occur much more sparingly 
than the true basalts. They are found in the Thuringer Wald, 
Erzgebirge, Baden, &c. 
(3) Leucite Rocks. 
This division includes certain grey or black crystalline or 
compact volcanic rocks resembling some of the basalt series, but 
distinguished from them by the predominance of leucite. The 
more crystalline-granular varieties, named leucitophyre or 
leucite-porphyry, are composed of a characteristically dull 
grey aggregate of leucite, augite, and magnetite, with some- 
times a little nepheline, olivine, or mica. The leucite occurs in: 
well-detined garnet-like crystals of a dull white colour, sometimes 
an inch in diameter, not infrequently broken and with fissures 
interpenetrated by the surrounding ground-mass, The rock is 
* On gabbro, see Lang, Z. Deutsch. Geol, Ges. xxxi. p. 484. 
