2g6 Dr. Holland on the manufacture of 
the front of which is deeply worn out, so as to show the stra- 
tification of the primitive slate, mixed with chlorite slate, with 
the other magnesian minerals just noticed, and with nume- 
rous veins, or layers of copper and iron pyrites. The speci- 
mens of this pyrites have a steel grey, or greenish yellow 
colour, according to the respective predominance of iron or 
copper. The whole substance of the pyrites has a schistose 
structure, corresponding with that of the rocks in which it is 
situated. It is so intimately mixed with the same magnesian 
minerals, as to be for the most part extremely unctuous to 
the touch. It is in general easily sectile. The specific gra- 
vity varies from 3.6 to 4.6. 
It is on this side of the Monte della Guardia, and from the 
materials just described, together with the magnesian lime- 
stone of Monte del Gazzo,that the manufacture of sulphate of 
magnesia has been established ; it is at present carried forward 
on a small scale, but is evidently capable of extension and 
improvement. The original object of attention in this place 
was the working of the copper and iron pyrites ; but the obser- 
vation of the crystals of sulphate of magnesia, formed during 
the processes applied to these ores, changed in great degree 
the character of the manufacture ; and the green vitriol and 
copper obtained here are now become secondary objects to 
the proprietors. The process employed for the formation and 
separation of the sulphate of magnesia is briefly as follows : 
the pyrites is worked out of the mountain by tunnels, 
which the steep natural section of its front, and the general 
direction of the veins, allow to be carried nearly on an hori- 
zontal level. Some of these galleries, which I saw, were 
more than 200 feet in length, and varying from 10 to 15 feet 
