STRUCTURE AND DISTRIBUTION OF DA CITES. 263 



Dacite. 



Dacites are quartz-hornblende andesites. They were associated 

 with quartz-trachytes till distinguished by Stache. Their colour is 

 always of a blackish grey-green or dark-brown. The texture is either 

 compact or granular, and typically the rock consists of quartz, oligo- 

 clase, and hornblende, probably with a second felspar, and occasionally 

 mica, with which may be associated a little augite and olivine. Dacites 

 differ from andesite in the character of the ground mass, which when 

 present in dacite has a rhyolitic structure with a tendency to form 

 spherulites, while the andesite ground mass is an aggregate of 

 microliths. 



Hungarian Dacites. In Hungary and Transylvania the dacites 

 have no glassy base, and have been divided by Doelter according to 

 their resemblance to granite, trachyte, and porphyry. Some, as at 

 Rodna and the Vlegyasza Mountains, have no true ground mass, but 

 a fine-grained matrix, with isolated larger crystals, and present the 

 conditions of granite. Near Roclna, biotite is sometimes so abundant 

 as to form a biotite dacite, and the trachyte dacite is generally rich 

 in biotite and poor in sanidine, with scattered particles of quartz, 

 which, however, does not occur here in the ground mass. In the 

 neighbourhood of Nagyag the porphyritic dacites take on the aspect 

 of quartz-porphyries. The dacites around Kapnik often contain small 

 masses with a glassy base, and frequently include pyrite. Timazite 

 is a rock sometimes to be classed with the dacites, and sometimes 

 with the andesites. At Les Crottes, in the Department of Var, the 

 dacite has a granitic texture, while at St. Raphael the rock is micro- 

 felsitic. Both these dacites are rich in sanidine, the crystals some- 

 times being 20 mm. long, so that the rock approximates towards 

 rhyolite. 



Dacites also occur at Neu Prevali in Carinthia and Monte Alto in 

 the Euganean Hills. Among the specimens which have been analysed 

 are examples from Besobdal, Ararat, Kasbek, Ebendaher, and between 

 Iveshet and Kobi. A dacite from the volcano of Mojanda in Ecuador 

 has a microcrystalline ground mass which includes rounded grains of 

 quartz, andesine, hornblende, and biotite. 



American Dacites. In the eastern half of the Cortez range 

 dacite forms a nearly continuous field for a distance of fourteen or 

 fifteen miles, but behaves like granite, forming massive eruptions 

 15,000 feet thick, and shows no tendency to extend itself laterally 

 from the region of fissure or to form sheets. Its prevailing character 

 varies, and the colour varies from purple to chocolate and brown, 

 though growing pale to the north of Waggon Canon, where the tinte 

 are olives and greys. At the south it is covered with rhyolite, and 

 on the east is overlain for many miles by basalt. The behaviour of 

 this rock in the field is more like propylite than andesite, and in hand 

 specimens it is not easily distinguished from quartz-propylite. Dacite 

 wants the resinous lustre and glassy fracture of the andesites. The 



