226 HISTORY OF DIORITE. 



the hornblende, but biotite is often a product of decomposition of the 

 green hornblende. The commonest result of decomposition is a fibrous 

 structure in the hornblende, which is often more or less converted 

 into epidote, though chlorite is the commonest decomposition product. 

 At the Lac d'Aydat it is altered into a substance like serpentine. 



When quartz is well developed it always resembles the quartz of 

 granite, and often contains fluid inclusions and microliths ; but it is 

 sometimes a secondary product. Augite is only found in diorites 

 with fibrous hornblende ; it occurs in clear reddish-brown grains, 

 which are readily converted into chlorite. Garnets occur in diorite 

 near Freiberg and at Aschaffenburg. 



Typically the rock often has a true granitic structure ; but some- 

 times, as in the Yosges, Southern Schwarzwald, and parts of Cornwall, 

 it assumes a slaty texture, as though it were a metamorphosed slate. 

 The orbicular diorite of Corsica has a concentric concretionary struc- 

 ture, and is rich inanorthite ; an approach to this structure is recorded 

 from Pondieres, in the Auvergne, though in the former locality the rock 

 contains augite, and in the latter hornblende. The best-known quartz 

 diorites occur at Quenast in Belgium, and Catanzaro in Calabria. 

 In the latter locality it consists of oligoclase, hornblende, augite, mica, 

 quartz, and chlorite, and has a porphyritic structure. Tonalite may be 

 classed as a quartz-mica diorite. Ophite differs in no respect from 

 ordinary diorite, but a Spanish example contains a true amorphous 

 base, as well as gas and fluid inclusions in the crystals. Eosenbusch 

 regards porphyrites as porphyritic rocks, which would otherwise be 

 classed as mica-diorites. Teall refers the Scotch porphyrites to the 

 andesites. 



North-American Diorites. Among the more typical diorites l of 

 the Virginia range is Mount Davidson, 7827 feet high, which is 

 surrounded by the volcanic rock termed propylite. In this diorite 

 the plagioclase is often fresh, and has fluid inclusions. There is no 

 orthoclase, but a good deal of quartz. The hornblende is dark-green, 

 fibrous, and more or less altered, the interstices in it being filled with 

 epidote and calcite. Sometimes quartz occurs in considerable quantity. 

 There are small exhibitions of diorite in Washoe, in basalt canon, 

 others in the Peavine Mountain in Nevada, and in Truckee canon, 

 where the rock resembles that of Ilmenau in the Thuringerwald. 

 Some of these diorites are poor in hornblende and biotite, and rich in 

 quartz and apatite. At the Hot-Spring Hills in the Pah-Ute range, 

 the diorite consists almost entirely of plagioclase, with veins and 

 spots of hornblende and magnetite. Many localities in Nevada pre- 

 sent slight differences in composition, but there is rarely anything 

 remarkable in its mineral composition, though at the foot of the 

 Augusta Mountains tourmaline appears to be a constituent, and the 

 granular rock of Winnconneca Peak contains dihexaedral quartz, en- 

 closing cubes of salt in the fluid inclusions, like those seen in the 

 Belgian diorite from Quenast, used for paving Paris. 



An exceptional variety of dioritic rock occurs as a dyke in granite 

 1 Zirkel : Micros. Petrog. 



