CHEMICAL COMPOSITION OF TRACHYTE. 



265 



fragments conspicuous for their irregular corners and angles ; and not 

 ^infrequently complete sanidine crystals are seen, when polarised, to 

 be a cemented agglomerate of angular sanidine, so that two pro- 

 cesses of crystallisation appear to have taken place in the rock in 

 succession, one dating perhaps before the eruption, and the other 

 after it. At Capo Negro in Ischia, the reddish lavas have the 

 sanidine in parallel threads, but the crystalline arrangement is some- 

 times in concentric strips, and a crystal often shows zones of growth 

 in the interior. 



The plagioclase probably includes several different felspars. 

 Tschermak has proposed to distinguish plagioclase of glassy texture 

 in andesites as microtine. The sanidine is frequently invested by 

 plagioclase ; and this condition is well seen in the trachyte of Battag- 

 lia, in the Euganean Hills. 





Hornblende sometimes occurs in well-developed crystals, some- 

 times in ill-defined grains. It is distinguished with difficulty from 

 augite ; the cleavage being the only satisfactory means of differentia- 

 tion, and in some small columnar crystals this is absent. Hornblende 

 in trachyte is almost invariably dark brown, but is green at Mocsar 

 and Tepla in Hungary ; and is often green when found in micro- 

 liths. It is sometimes enveloped in magnetite, and may contain 

 microliths of apatite, and glass and gas inclusions. In the ground 

 mass, hornblende occurs in bundles of small needles, like those seen 

 in phonolite. Hornblende occurs alone in the trachytes of the 

 Azores, at Kieshiibel near Schemnitz, and at Kuhlsbrunnen in the 



1 U. S. Geol. Surv., Fortieth Parallel, vol. i. p. 604; Table X. 



