Cn. xxviil] trachyte— clinkstone. 599 



proportion of magnesia than serpentine, it has been suggested with 

 much probability that in the course of ages some basalts highly 

 charged with olivine may be turned, by metamorphic action, into 

 serpentine. 



Trachyte. — This name, derived from rgaxvg, rough, has been given 

 to the felspathic class of volcanic rocks which have a coarse, cellular 

 paste, rough and gritty to the touch. This paste has commonly been 

 supposed to consist chiefly of albite, but according to M. Delesse it is 

 variable in composition, its prevailing alkali being soda. Through the 

 base are disseminated crystals of glassy felspar, mica, and sometimes 

 quartz and hornblende, although in the trachyte, properly so called, 

 there is no quartz. The varieties of felspar which occur in trachyte 

 are trisilicates, or those in which the silica is to" the alumina in the 

 proportion of three atoms to one.* 



Trachytic Porphyry, according to Abich, has the ordinary compo - 

 sition of trachyte, with quartz superadded, and without any augite or 

 titaniferous iron. Andesite is a name given by Gustavus Rose to a 

 trachyte of the Andes, which contains the felspar called Andesin, 

 together with glassy felspar (orthoclase) and hornblende disseminated 

 through a dark-colored base. 



Clinkstone, or Phonclite. — Among the felspathic products of vol- 

 canic action, this rock is remarkable for its tendency to lamination, 

 which is sometimes such that it affords tiles for roofing. It rings 

 when struck with the hammer, whence its name ; is compact, and 

 usually of a grayish blue or brownish color ; is variable in composition, 

 but almost entirely composed of felspar, and in some cases, according 

 to Gmelin, of felspar and mesotype. When it contains disseminated 

 crystals of felspar, it is called Clinkstone porphyry. 



Greenstone is the most abundant of those volcanic rocks which are 

 intermediate in their composition between the Basalts and Trachytes. 

 The name has usually been extended to all granular mixtures, whether 

 of hornblende and felspar, or of augite and felspar. The term diorite 

 has been applied exclusively to compounds of hornblende and felspar. 

 According to the analyses of Delesse and others, the chief cause of 

 the green color, in most greenstones, is not green hornblende nor 

 augite, but a green siliceous base, very variable and indefinite in its 

 composition. The dark color, however, of diorite is usually derived 

 from disseminated plates of hornblende. 



The Basalts contain a smaller quantity of silica than the Trachytes, 

 and a larger proportion of lime and magnesia. Hence, independently 

 of the frequent presence of iron, basalt is heavier. Abich has there- 

 fore proposed that we should weigh these rocks, in order to appreciate 

 their composition in cases where it it is impossible to separate their 

 component minerals. Thus, the variety of basalt called dolerite, which 

 contains 53 per cent, of silica, has a specific gravity of 2*86; whereas 



* Dr. Daubeny on Volcanoes, 2d ed. pp. 14, 15. 



