a = 
30 Descriptive Arrangement of Volcanic Rocks. 
a 
view is to offer concise and comprehensive definitions of the 
principal oryctognostical characters of this family of rocks, 
e use of which any one of its members may be distinct- 
ly described by an observer in a manner intelligible to all 
geologists. 
The primary characters by which alone the nature of a 
rock or mineral mass, simple or compound, can be identified, 
teristics, € primary qualities, that of mineral composi- 
tion 1s obviously by far the most important towards identify- 
ing the rock. All the other characteristics are pro ac- 
cidental modifications determined by the mineral composi- 
tion, under the influence of external circumstances ; whereas 
it 1s difficult to conceive this latter character to be’ in any 
way influenced by the others, under any circumstances. _ 
Hence the mineral composition of the rocks under review 
at present has been taken as the basis of their systematic ar- 
rangement into genera and species ; the sub-species and va- 
3; the consequence of which would be, that we should have no names 
im 
say that granite k determined by its underlying gneiss ; 
g by its underlying mica-schist, and by its beari 
k dso wo [ ursue the most vicious of 
all cire! eans of nguishing an over fi 
derlying roe by thei ions of mineral character: these mu 
Crick Geel peepee ralogi teristics of each roc! 
jand the mine cs of each rock defined, 
: without ing some name to it, ore their rela- 
ns of position can become a question. To force the pri : k 
arises a gener: 
> of its mineralogical character. : 
; qu name of a roc 
to denote its supposed place in a ical series, would be as inconvenient 
irrational as to confine the name of a simple mineral to one found in a par- 
ieular locality, so that, when met with in new nam in- 
Vented for it; or to give, for » the name of felspar to this mineral onl 
Heompany with mica; and refuse it the appellation when associated with 
it is si 
