228 Dr. Cooper on Volcanoes 
black, and blackish varieties, where the hammer has struck 
it. ‘The aspect when bruised is white and dusty. 
The compact fine grained variety is hard and very tough ; 
hence it is used for the gudgeons of mills to run it; bearing 
a fine polish. But it is often porous and hackly to the 
touch, as well as smooth and compact. Generally it gives 
fire with steel ; a property attributable to the accidental 
presence of silex, or to the indurating effect of fire. Gene- 
rally also it acts on the needle ; itis something polar: but 
these properties depend much on the quantity and state of 
the iron contained in it, and vary in different specimens 
and often in the same specimen. ‘The iron is usually in the 
state ofa black oxyd within, and yellow oxyd without. 
It is fusible before the blow-pipe ; more easily in pro- 
portion to the felspar it contains. In a glass furnace, it runs 
into a greenish or greenish black, bottle glass, adhering 
with some difficulty to the glass blowers’ rod: the colour of 
the glass is lighter in proportion to the felspar contained in 
the stone. 
It is not acted on by acids. 
Basalt is always massive, sometimes stratified. The 
masses assume different forms : sometimes they form high 
and rugged peaks : often they are prismatic and columnar, 
with four or five sides ; less frequently with three or six, 
seven or eight: occasionally but very rarely with nine. 
This figurate basalt, as it is often called, has the external 
appearance of crystallization ; but the sides are not of any 
determinate number ; the angles have no regularity in their 
dimensions ; the inside of the stone has no distinct crystal- 
lization in the mass; no nucleus or primitive crystal can 
be traced by means of any natural joint or fracture. Hence, 
the form of the prismatic figurate or columnar basalt, has 
been ascribed to the contraction that takes place on cooling. 
in a heated or mm drying, in a moist mass. 
There are also shistose (feuilleté Fauj. St. Fond. 156) 
tabular, and globular basalts ; it is found also concentric- 
lamellar. The two last varieties approach so much to 
crystallization, that the arguments of M. Patrin, who as- 
cribes the columnar form to this property are difficult to 
be refuted, though not generally adopted. The experi- 
ments of Mr. Gregory Watt, shew, that fused basalt some- 
times crystallizes. in spheroidal masses ; and that figurate 
