“it 
Descriptive Arrangement of Voleanie Rocks. 33 
state, onder’ which circumstances the Tightes colored . 
rocks sometimes assume a blackish h 
that avery fre per cent.; when thes ‘initials are in less 
ortion than seventy- Ave per cent., the rock should be 
~ elassed as basalt. 
Another auxiliary test, in which greater confidence may 
be reposed, is the specific gravity of the substance when re- 
duced to powder. In fact, the specific gravity of the augitic 
and ferruginous minerals is so greatly superior to that of fel- 
spar, that an observation of this nature will indicate the gen- 
eral proportion of these two classes of minerals in any vol- 
canic rock. In general, the specific gravity of ‘ trachyte will 
be found not to exceed 2. 7, that of greystone 3.0, while ba- 
salt occasionally reaches 3.50, which is much above the spe- 
cific gravity of augite alone, and caused sf the presence of 
a quantity of iron in a metallic state. _ 
A third test consists in the colour of the glass, Bemaisas 
by fusion of the mineral before the blowpipe. That resulting 
from trachyte is light coloured, and nearly transparent. 7 
glass of greystone is darker, and spotted with numerous 
green or black specks, often of a green colour, bearing a 
Basalt melts into a dark green, or black oat 
Observations which have often been made on these r 
and w without being insisted on as infallible criteria 
will yet often assist in distinguishin, ing them, are— 
1. That leucite has n rom found to occur in any tra- 
chyte, only making its mare ated when the proportion of 
the heavier minerals is considerable; rarely in greystone, 
- oftener in basalt. : 
* Appa rently derived from the ae which appears, from chemical anal- 
Mora Seca ait —~ in this condition of the rock, and which is volatilized on ex- 
posure to the blowpipe. 
Vor. XV.—No. 1. 5 
