32+ Descriptive Arrangement of Volcanic Rocks. 
uate so imperceptibly into one another, that two genera alone 
can hardly be reckoned sufficient. : 
very numerous tribe of rocks is to be met with, in which 
the proportions of felspar and augite are such, that it is im- 
possible to tell which predominates, while, in their general 
characters, the rocks are so unlike the extremes of either ge- 
nus, that it is scarcely allowable to rank them together. It 
s from these reasons expedient to institute an inter- 
mediate class of rocks, for the reception of those members 
which cannot, without difficulty, be referred to either of the 
two extremes. ‘The name which I conceive most appropri- 
ate to them, as having been applied to such rocks already 
by Werner and other mineralogists, and as susceptible of lit- 
tle misconception, is greystone, (graustein,) their color being 
universally of some tint of grey, generally lead-grey, green- 
ish, tron, purplish, or slate-grey, with the exception only of 
their vitrified varieties, some of which have assumed a black 
color, which, however, passes away under the blowpipe, and 
is succeeded by the usual grey tint.* 
The genera of the volcanic, or trap family of rocks, will 
then consist of—_ 
I. Trac : 
Il. Greystone. 
III. SALT. 
It may, perhaps, be. objected to the basis of this arran 
ment, that slinee ubjects fre eal deucres, 
and hence their constituent minerals are undiscoverable. 
is, er. the case. There are very few 
“hest, a microscope, will not discover a granitoidal mixture 
of the constituent minerals in a crystalline form. The meth- 
od of hechanical analysis, first proposed by M. Cordier, will 
det ne thi with i for di 
nary purposes, examination with a lens will be sufficient, and 
even the color ae gst aang ly depended on as an accurate 
criterion, unless the is passing to a resinous or vitreous 
* Gre tone corresponds in part to the class of volcanic rocks called te- 
Pliny Bt dol Meter Xt campus he ape fe 
