278 



VOLCANIC ROCKS. TRACHYTE. 



have flowed as lava, and which appear the most homogeneous, are 

 composed of microscopic crystalline particles, belonging to a small 

 number of minerals, particularly felspar, augite, olivine, and iron 

 sand ; and the same intermixture of minerals may be observed in 

 all scoriaceous lava and in basalt. To the white or grey lava, com- 

 posed principally of felspar, the French have given the name of 

 trachyte^ from its breaking with a rough surface. 



Trachyte.— Common or stony trachyte has generally a whitish or 

 greyish colour, a dull earthy fracture, and is more or less fine-grained; 

 sometimes the grains are very minute; it has then a compact surface, 

 and sometimes a glistening lustre, in which state it becomes pearl- 

 stone. Its hardness is variable ; some of the trachytes near Cler- 

 mont are spongy, and almost friable. Trachyte melts readily into a 

 greyish glass ; it generally contains imbedded crystals of vitreous 

 felspar. Acicular or needle-shaped crystals of hornblende, hexa- 

 gonal crystals of mica, and grains of iron sand, and laminae of spec- 

 ular iron ore, occur in trachyte. Augite is seldom found in the tra- 

 chyte of Europe, though it is common in the trachytes of the Andes. 

 The claystone of Braid Hill, near Edinburgh, nearly resembles some 

 of the trachytes in Auvergne ; but it is not porphyritic. Trachyte 

 may be regarded as an earthy form of felspar ; it is, therefore, un- 

 necessary to speak of its constituent parts. To the variety of tra- 

 chyte on the Puy de Dome, M. Von Buch has given the name of 

 domite^ — a term which the French geologists have properly rejected, 

 as it is only common trachyte, rather whiter than some of the other 

 varieties. It has before been stated, that the trachytes in Auvergne 

 were probably formed by the more or less perfect fusion of granite ; 

 like the granite of that district, they contain but a very small portion 

 of quartz. 



Trachyte occurs in the Lipari Islands, in a perfectly vitreous state, 

 forming obsidian or volcanic glass, which is sometimes colourless, and 

 sometimes black ; the black variety, however, forms a white glass 

 when melted. The colouring matter, being fugitive, is probably bi- 

 tumen : in this respect, it differs from obsidian formed from dark 

 lava or basalt : the latter melts into a black glass. Pumice appears 

 to have been formed from felspar or trachyte, exposed to an intense 

 heat, which has reduced it to a fibrous mass. 



The island of Lipari contains a mountain formed entirely of white 

 pumice : when seen at a distance, it excites the idea, that it is cov- 

 ered from the summit to the foot with snow. Almost all the pumice- 

 stone employed in commerce is brought from this immense mine. 

 The mountain is not one compact mass, but is composed of balls or 

 globes of pumice aggregated together, but without adhesion. From 

 hence Spallanzani infers, that the pumice was thrown out of a vol- 

 cano in a state of fusion, and took a globose form in the air. Some 

 of these balls of pumice do not exceed the size of a nut, others are 

 a foot or more in diameter. Many of these pumices are so compact, 



