ITALY.. LETTER XV. 207 



only more or lefs mixed with clay. The argilla- 

 ceous (late, N 5. (and perhaps fimilar hills on 

 both fides of the way) is entirely defticute of 

 any calcareous mixture. So are likewifc the 

 alum- rocks, N 7, which contain fo trifling a 

 quantity of lime, that it cannot be confidered as a 

 fubftantial part ; joined to the acid of vitriol it 

 precipitates as feienite. I could not obferve the 

 pofition of thefe feven different forts of ftone ; 

 whether their difference is owing only to a differ- 

 ent mixture in the furface ; or, whether the argil- 

 laceous Hate, and the white alum-rock, as in other 

 parts of Italy, be inferior to the fuperincumbent 

 calcareous hills; and but in fome places appear 

 above ground, either bare and unmixed, or as 

 marl, more or lefs calcareous. This laft I hold to 

 be moft probable, as obferved in many other parts 

 of Italy; but, as the bad weather and fhortnefs of 

 time did not permit me any clofer examination o 7 

 this country, I could not afcertain it, and leave it 

 therefore to your future enquiries. 



In argillaceous (late hills;, mentioned N 5, are 

 to the right, and at fome didance old deferred 

 lead, copper, and iron- mines ; and probably the 

 celebrated quartz-cryflallizations, going under the 

 name of Tolfa-diamonds, may be found in thefs 

 mines ; but as the alum-rocks, N 7, offer like- 

 wife quartz- fiffures, I cannot be pofitive about it. 



Amtt?s 



