34 Descriptive Arrangement of Volcanic Rocks. 



2. Olivin never has been met with as yet but in basalt ; it 

 appears to replace the felspar, in part or altogether, only 

 when augite is in excess. 



The specific divisions of these genera should be drawn 

 from minor modifications of mineral constitution : a tabular 

 view of the principal species is subjoined : 



Genus I. — Trachyte, characterized as above. 



Species A. Compound trachyte with mica, hornblende, or 

 augite, sometimes both, and grains of titanife- 

 rous iron. 



" B. Simple T., without any visible ingredient but fel- 

 spar. 



" C. Quartziferous T., when containing numerous 

 crystals of quartz. 



" D. Siliceous T., when there appears to have been 

 introduced a great deal of silex into its com- 

 position. 



Genus II. — Greystone, 



" A. Common greystone, consisting of felspar, augite. 



or hornblende and iron. 

 " B. Leucitic greystone, when leucite supplants the 



felspar. 

 " C. Mehlitic greystone, when melilite is substituted 



for that mineral, &,c. 



Genus III. — Basalt, characters as above. 



'^ A. Common basalt, composed of felspar, augite, and 



iron. 

 " B. Leucitic B., when leucite replaces the felspar. 

 " C. Basalt, with olivin in lieu of felspar. 

 " D. Basalt, with hauyne in heu of felspar. 

 " E. Ferruginous basalt, when iron is the predominant 



ingredient. 

 " F. Augitic basalt, when pyroxene or hornblende 



composes nearly the whole of the rock. 



The character which ranks next in importance, towards 

 the descriptive qualification of a volcanic rock, is its texture, 



