ON THE GEOLOGY OF THE 
If the Conway basin be thus interesting to the lover of 
scenery and the agriculturist, it is no less so to the practi- 
cal mineralogist. It possesses in different places valuable 
mineral treasures of great importance in the arts. 
Of these, slate may be considered as the most abundant. 
The general nature of this rock, however, not being very 
hard, no great quantity of it is found fit for roofing-slate. 
The best for that purpose is what is raised on the banks 
of the Lledar, above Dolwyddelan Castle, and to the east 
of Snowdon. It may be remarked, that the slaty rocks 
being here very hard, good roofing-slate is accordingly 
raised. It is conveyed in carts upwards of twelve miles, 
and, being shipped at Trefriew, forms a considerable ar- 
ticle of export. Many thousand tons are annually shipped. 
But the colour of the slate being reddish, is by some con- 
sidered objectionable, and from this prejudice a less quan- 
tity is sold than might be expected. No other slate, fitted 
for architectural purposes, is elsewhere worked, except 
near Trefriew ; and its sale is very limited, being not so 
good as the Lledar slate, and very inferior to what we 
shall afterwards find on the other side of the Snowdon 
range. 
The slate is frequently penetrated on the western side 
of the basin by dikes of whinstone, of no great thickness ; 
and it is invariably found where these occur, that the slate 
in immediate contact with them becomes harder and touo^her. 
This trap-rock being so conveniently dispersed, is employed 
as the common material for road-metal. 
Of quartz, or buhrstone, a considerable quantity is found 
within a mile, and to the west, of the town of Conway. 
The quartzy buhrstone has occasionally been employed as a 
millstone, but is not found to answer so well as that which 
comes from France. It is imbedded in a soapy aluminous 
stone, and forms the eastern point of Penmacn Bach. The 
