
140 GEOGNOSY. +S |Beow ies 
appears as a crystalline aggregate of plates of sanidine and hexagonal 
prisms of nepheline with less frequent crystals of leucite, hornblende, 
augite, magnetite and hauyne. The rock is rather subject to 
decomposition, hence its fissures and cavities are frequently filled 
with zeolites. An average specimen contains silica, 57°7; alumina, 
20°6; potash, 6:0; soda, 7:0; lime, 1:5; magnesia, 0°5; oxides of 
iron and manganese, 3°5; loss by ignition, 3:2 per cent.” The 
specific gravity may be taken as about 2°58. Phonolite is sometimes 
found splitting into thin slabs which can be used for roofing purposes. 
Occasionally it assumes a porphyritic texture from the presence of 
large crystals of sanidine or of hornblende. When the rock is partly 
decomposed and takes a somewhat porous texture, it resembles 
trachyte in appearance. 
Like trachyte, phonolite is a thoroughly volcanic rock and of 
Tertiary date. It occurs sometimes filling the pipes of volcanic 
orifices, sometimes as sheets which have been poured out in the form 
of lava-streams, and sometimes in dykes and veins, as in Bohemia 
and Auvergne. 7 
Pitchstone (Retinite)—A vitreous, pitch-like rock easily 
frangible, translucent on thin edges, having usually a black or dark- 
green colour that ranges through shades of green, brown, and yellow 
to nearly white. It is essentially an orthoclase rock, and may be 
regarded as the natural glass resulting from the rapid cooling of 
many of the more granular or crystalline orthoclase rocks, such as 
the quartz-porphyries or felsites. xamined microscopically, it is 
found to consist of glass in which are diffused, in greater or less 
abundance, hair-like microliths, angular or irregular grains, or more — 
definitely formed crystals of orthoclase, plagioclase, quartz, &c. The 
pitchstone of Corriegills, in the island of Arran, presents abundant 
green, feathery, and dendritic microliths of a pyroxenic character 
(Fig.9). Occasionally, as in Arran, pitchstone assumes a spherulitic 
or perlitic structure. Sometimes it becomes porphyritic by the 
development of abundant sanidine crystals (Isle of Higg). | 
According to Durocher the mean composition of pitchstone is— 
silica, 70°6; alumina, 15:0; potash, 1:6; soda, 2°4; lime, 1:2; 
magnesia, 0°6; oxides of iron and manganese, 2°6; loss by ignition, 
6:0. Mean specific gravity 2°34. 
Pitchstone is found as (1) intrusive dykes, veins, or bosses, 
probably in close connection with former volcanic activity, as in the 
ease of the dykes which in Arran traverse Lower Carboniferous 
rocks but are probably of Miocene age, and those which in Meissen 
send veins through and overspread the younger Palmozoic felsite- 
porphyries; (2) sheets which have flowed at the surface, as in the 
remarkable mass forming the Scuir of Higg which has filled up a 
river-channel of Miocene age.! 
Obsidian.—A volcanic glass representing the vitreous condition 
of a sanidine-rock, such as trachyte or liparite. It externally resembles 
* Quart. Journ. Geol. Soc. 1871, p. 303. 
