and Volcanic Substances. 227 
And first, let us describe basalt ; wherein, if profess to 
give the most usual discriminating characters of this Rock, 
and I offer i it, as equally true of the basalt of acknowleitged 
voleanic origin, and of the basalt of disputed eS and 
80 submit it to the mineralogical reader. In fact, as Brong- 
niart, (who in 1807, was a vulcanist,) says, it is difficult to 
explain la presence presque habituelle des Basaltes prisma- 
peg dans les pays evidemment volcaniques. Min. v. i, p. 
aa whenever found as a Lava, or asa member of the 
floetz trap formation, is a compound stone of sp. gr. from 
2,85 to 3. When exposed to the air, generally covered 
with an ochry crust on the outside. Withinside, it is of a 
blackish brown, a greyish or a greenish black according to 
the predominance of felspar in its mixture id ue eee 
blende or augit, that constitute with oxyd of ir 
maining bulk of the ‘a, When polished sat gery it 
assumes a bluish as 
t is sometimes porphyriti with crystals, or with ob- 
tuse, oblong, or rounded grains of felspar. It is sometimes 
stellated, as the Abbe Fortis found it in the Vincentin. It 
is very often amygdaloidal, (as the same observer remarks,) 
with carbonat of lime, as at Castagansnote:s and as the 
Itis often granular: but the component parts are some- 
times so intermixed as to appear homogeneous ; at other 
times the component parts are distinguishable lh the naked 
eye. When viewed steadily ae a good magnifying 
glass, the black portion appears in small dull, irregular, 
short crystals, whose angles are scaerally obliterated, 
They are not flat and shining like the crystals of augit or 
even of hornblende. The white part of the grey and black- 
ish varieties is felspar, often petrosiliceous in its aspect. 
When bere | into minute fragments, (not dust,) according 
to Cord directions, the character of the c component 
yes is nach more apparent through the microscope. 
racture of the compact varieties tends to conchoi- 
dal ; with an approach to radiation, especially in the grey 
