

NATURAL HISTORY OF RHYOLITE. 269 



nepheline and leucite. Nepheline is often as important as sanidine 

 in the ground mass. It may give six-sided or quadrate sections. 

 This mineral is fresh and clear as water, and not easily determined. 

 Like leucite, which also enters the ground mass, it may occur in 

 microscopic crystals. Plagioclase occurs in the phonolites of the 

 Auvergne, which are poor in nepheline, and is rare in the phonolites 

 of the Rhb'n and the Kaiserstuhl. Nosean and hauyine may be absent 

 from phonolites which contain no nepheline ; but usually both these 

 minerals are more or less important. Hauyine may be macroscopic as 

 well as microscopic : its colour is variable, black, brown, blue, yellow, 

 green, or colourless. It is rare for hornblende to occur, unless 

 associated with augite, and frequently the augite predominates. The 

 larger crystals of hornblende may be brown or green. Hornblende- 

 bearing phonolites occur at Teplitz and Aussig, and at Spansdorf 

 and Grosspriesen, and on the road from Mont Dore to Murat, &c., 

 hornblende occurs with much green augite. Both those minerals 

 may occur as porphyritic crystals or as part of the ground mass. 

 More rarely augite is present without any hornblende. The leucitic 

 rocks are richest in augite. Many phonolites in Cantal and Heidel- 

 berg contain magnesia-mica. The apatite is blue and brown, and 

 may form thick prisms. Titanite is either greenish yellow, clear 

 yellow, or orange-red. Tridymite is common in the phonolite at 

 Aussig. Olivine occurs at the Roche Sanadoire, and some other 

 minerals occur exceptionally in other localities. The base is often 

 globulitic, as at Hohenwiel. It may be amorphous or micro-felsitic 

 in some places, or occasionally granitic. A phonolite-like obsidian is 

 found at Teneriffe. Occasionally when the rock has a trachytic character 

 it is porous, and often has the cavities filled with zeolites. Spotted 

 phonolites are found in Teneriffe. The phonolites include seven 

 mineral varieties, which may be grouped as nepheline-phonolite, 

 nosean or hauyine phonolite, and sanidine-phonolite. 



Rhyolite. 



The name rhyolite was first used by Yon Eichthofen in 1860, to 

 define Hungarian and Transylvanian lavas, which consist of crystals 

 of quartz and sanidine, scattered through a felsitic ground mass. A 

 similar rock is well seen in the Lipari Islands, from which it was 

 named Liparite, by Justus Roth, in 1861. It has been found in the 

 Euganean Hills, in Rhenish Prussia, the Auvergne, Iceland, the 

 Rocky Mountain region of North America, the Northern Island of 

 New Zealand, and several of the islands of the Greek Archipelago. 



Rhyolites are the volcanic equivalents of granites, and are identi- 

 cal with quartz porphyry, quartz felsite, and felsitic pitchstone, associ- 

 ated with the Primary strata. 



No volcanic rock presents greater varieties of texture and micro- 

 scopic structure than rhyolite. Some rhyolites are entirely crystal- 

 line ; others have crystals in greater or less quantity scattered through 

 an amorphous base ; while a third type is absolutely free from 



