DELESSE ORIGIN OF ROCKS. g 



§ 16. Trachyte. — This is a rock which, according to M. Nauinann, 

 contains orthose, anorthose, ferro-magnesian mica, and hornblende. 

 Quartz also occurs in it. These minerals are disseminated in a 

 cellular and rough paste, which is generally of a grey, white, or 

 reddish colour. The orthose is splintery, transparent, and has a 

 bright vitreous fracture which gives it a peculiar character ; it con- 

 tains numerous fissures parallel to the length of the crystals. It 

 differs so much from the nacreous orthose of granite, that mineralo- 

 gists consider it as a distinct mineral, and have given it the name of 

 vitreous orthose, or of ryakolite, or sanidine. 



The ferro-magnesian mica occurs in well-defined crystals of a 

 brilliant black, or very dark brown colour. It is never accompanied 

 by the silvery white aluminous mica which is found in granite, 

 gneiss, and mica-schist. 



The hornblende is black, and rarely of a blackish-green colour. 

 Its crystals are well formed, elongated, and even complete at both 

 extremities ; they have a bright glance, and their lamellar structure 

 is very characteristic. 



The quartz is rare when the trachyte is very cellular, viz. when 

 it bears evident traces of fusion. On the other hand it is abundant 

 when the trachyte has a more compact structure, and approaches 

 more nearly to quartziferous porphyry. Moreover the examination 

 of trachytic districts shows frequent passages between quartziferous 

 porphyry and trachyte containing quartz. 



The fissures of trachyte are often covered with crystals of specular 

 oligist-iron ; sometimes the rock is completely impregnated with 

 them. It also contains chlorhydric acid, sulphur, and sulphuric acid. 

 It is important to note the presence of these substances, as we see 

 rocks thrown out by modern volcanos impregnated with them. 



Some trachytes also contain zeolites : those of Hungary are also 

 often traversed by veins of opal ; they are then not quite anhydrous, 

 and consequently water has not been altogether absent in their 

 formation. 



With regard to its chemical and mineralogical composition, trachyte 

 is closely connected with granite, from which it differs principally 

 by its cellular structure and the vitreous glance of its constituent 

 minerals ; but in proportion as these characters disappear, a group of 

 intermediate rocks is produced which contain more and more quartz, 

 and in which the influence of heat appears to have been gradually 

 diminishing. 



Some writers, as Desmarets, de Saussure, Dolomieu, and L. von 

 Buch, have supposed that the action of the volcanos of Auvcrgnc on 

 the granitic rocks amongst which they have burst forth might have 

 produced trachyte. According to this hypothesis, trachyte would be 

 to a certain extent the product of granite re-heated and metamor- 

 phosed. 



After observing (p. 754) that pressure is not essentially necessary 

 to the formation of trachyte, and that it is often accompanied by 

 thick beds of conglomerate on a large scale, he sums up his observa- 

 tions on this rock thus : — 



