C i8 7 ] 
to this nation, where it has been perfected, are yet fo 
unknown in France, that there are even mechani- 
cians in that kingdom, who ferioufly doubt, whether 
the fire engine is any-wife ufeful. I fhall not here 
mention any thing of fome rare and curious foffils 
found in Britany, nor of the fquare flones of a parti- 
cular fpecies, on the formation of which the late M. 
de Robier, prefident of the parliament of Rennes, 
who had a magnificent collection of natural hiftory, 
employed his thoughts. I likewife (hall pafs over in 
lilence the marbles and the plumb pudding ftones, 
called in France cailioux de Rennes , from the vaft 
quantities of them found in the neighbourhood of 
that city. The chief and only fubjeCt I propofe in 
this differtation, is the generation or production of 
the terra Tripolitana, or Tripoli, of which there are 
great quantities, and of the bed: kind, in Upper Bri- 
tany. I more readily determine to give this illuftri- 
ous Society an account of my obfervations on this fub- 
jeCt, as not only the difcovery I think I have made 
thereon is very curious, but alfo that the generation of 
this earth has been hitherto utterly unknown, no one 
having, to my knowlege, explained, before me, in 
what manner it is produced. It is true, I heard in 
France, that a young gentleman, a native of Britany, 
had wrote fomewhat on this fubjeCt, and that his dif- 
fertation was to be inferted in a collection of mifcel- 
laneous papers, or loofe pieces ; but I never could 
fee his faid differtation, nor know what it contained ; 
therefore I am incapable of judging what it is. 
In the mountain de Poligne, called by the Bretons 
le Tertre gris, i. e. the grey hill, I think I have dis- 
covered the true origin of Tripoli. This mountain, 
B b 2 the 
