228 Dr. Cooper on Volcanoes 
black, and blackish ne where the hammer has struck 
it. The aspect when bruised is white and dust 
The compact fine vied 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 ; it is 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. Ina lass 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 = 
seven or eight: occasionally but very rarely with n 
This figurate basalt, as it is often called, has the pre" 
appearance of crystallization ; but the sides are not of any 
detariniinad number ; the angles have no by ara in their 
tabular, “ead eine basalts 3 it is fe d ‘also concentric- 
lamellar. The tw. : 
times crystallizes in spheroidal masses ; and that figurate 
