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the Breton name whereof is manifeftly derived from 
the greyifh colour of its fummit, is iituated in Upper 
Britany, near the inn of Roudun, on the road from 
Rennes to Nantes, a fmall league wide of Baru, and 
five leagues from Rennes. Obfervations and expe- 
rience are the only means, that can conduct us to 
certain knowlege in natural hiftory ; and it is conform- 
able to that axiom, that I mean, limply, to lay be- 
fore the Society what I obferved in that mountain, 
relating to the generation of Tripoli, and, at the fame 
time, to produce alfo the mod; authentic documents 
of nature, in order to prove, that the terra Tripolitana, 
or Tripoli, is probably only a wood wholly petrified , 
and afterwards calcined by the fubt err aneous fre. 
To give more weight to what I propofe, and to 
eftablifh this affertion, it will be neceffary to obferve 
here, that the mountain ofPoligne, in the interior of 
which the Tripoli is found, has been, and is perhaps 
yet, a volcano. Its colour, its form, its fiffures, and 
its flrata, prove it ; and the inhabitants of the neigh- 
bourhood declare, that they have formerly feen fire 
on its fummit at night ; but that, however, for many 
years pah, they have not perceived any more. This 
being granted, I have only now to give an account 
of what is to be obferved in the mountain itfelf, and 
to produce fpecimina of the different flrata of earth 
found therein, to afeertain the truth of my thefis. 
The ffratum of the true Tripoli, intirely calcined, 
lies from 50 to 60 feet depth ; the fample of it here 
produced is marked N° 1 . This ffratum is white ; but 
it fometimes has a caff of grey, and fometimes is of 
a reddifh hue, as the fample itfelf fhews. It is quite 
or fully calcined and converted jnto Tripoli, becaufe it 
