and Folcanic Substances. 224 
And first, let us describe basalt; wherein, I profess to 
give the most usual discriminating characters of this Rock, 
and I offer it, as equally true of the basalt of acknowledged 
voleanic origin, and of the basalt of disputed origin; and 
so 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- 
teques, dans les pays evidemment volcaniques. Min. v. i, p. 
472. 
Basalt, whenever found as a Lava, or asa member of the 
floetz trap formation, is a compound stone of sp. gr. from 
2,85 to3. 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 with the horn- 
blende or augit, that constitute with oxyd of iron, the re- 
maining bulk of the stone. When polished and melted it 
assumes a bluish aspect. 
It is sometimes porphyritic with crystals, or with ob- 
tuse, oblong, or rounded grains of felspar. It is sometimes 
stellated, as the Abbe Fortis found it inthe Vincentin. It 
is very often amygdaloidal, (as the same observer remarks, ) 
with carbonat of lime, as at Castagnamoro ; and as the 
toad-stone of England and Scotland often is, and as the ba- 
salt of mount Holyoke in Massachusetts is. It is thus found 
at Adtna, at Lipari. 3 Spal. tr. 228. In Iceland, 7 Edin. 
trans. 90. At Monte Somma. Breisl. § 707. 
It is often granular: but the component parts are some- 
times so intermixed as to appear homogeneous ; at other 
times the component parts are distinguishable by the naked 
eye. When viewed steadily through a good magnifying 
glass, the black portion appears in. small dull, irregular, 
short crystals, whose angles are generally 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 broken into minute fragments, (not dust,) according 
to Cordier’s directions, the character of the component 
parts is much more apparent through the microscope. 
The fracture of the compact varieties tends to conchoi- 
dal; with an approach to radiation, especially in the grey 
