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.—Tracuyre, characterized as above. 
Species A. ees trachyte with mica, hornblende, or 
augite, o somes bet h, and grains of titanife- 
rous 
“ _ B. Simple T.. without any visible ingredient | but fel- 
~ C. Quartziferous T., when containing numerous 
crystals of qu uartz, 
« _ P—. Siliceous T., when there appears to have been 
introduced a great deal of silex into its com- 
position, 
Genus I].—GreysTone. 
A. Common greystone, TT of felspar, augite, 
or hornblende and iron 
B. Leucitie greystone, when leucite pe Sens the 
t 
C. Melilitic greystone, when melilite is subiatiteted 
for that mineral, &c. 
Genus 1t-B seawy, characters as above, 
ati A. hae basalt, composed of felspar, augite, and 
ie Bice B., when leucite pe "sind the felspar. 
Basalt, with olivin in lieu of fels 
D. Basalt, with hauyne in lieu of Sclgnait 
E. Ferruginous basalt, when iron is the predominant 
feat. B. Augitic foe when n pyroxene or hornblende 
composes nearly the whole of the rock. 
“Phe thatacter which ranks next in importance, towards 
the descriptive qualification of a arent rock, is its texture, 
