Ch. XXVIIL] TRACHYTE PORPHYRY — CLINKSTONE. 467 



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 composi- 

 tion of trachyte, with quartz superadded, and without any augite or tita- 

 niferous 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 Phonolite. — Among the felspathic products of volcanic 

 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 gray- 

 ish 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. Ac- 

 cording 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 therefore pro- 

 posed that we should weigh these rocks, in order to appreciate their com- 

 position in cases where it is impossible to separate their component min- 

 erals. Thus, the variety of basalt called dolerite, which contains 53 per 

 cent, of silica, has a specific gravity of 2*86 ; whereas trachyte, which has 

 66 per cent, of silica, has a sp. gr. of only 2-68 ; trachytic porphyry, con- 

 taining 69 per cent, of silica, a sp. gr. of only 2*58. . If we then take 

 a rock of intermediate composition, such as that prevailing in the 

 Peak of TenerifFe, which Abich calls Trachyte-dolerite, its proportion of 

 silica being intermediate, or 58 per cent., it weighs 2 - 78, or more than 

 trachyte, and less than basalt.f The basalts are generally dark in color, 

 sometimes almost black, whereas the trachytes are gray, and even occa- 

 sionally white. As compared with the granitic rocks, basalts and tra- 

 chytes contain both of them more soda in their composition, the potash- 

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



