TALY. LETTER XIII. lS$ 



this gave me an opportunity to make a mod in- 

 ftruding obfervation. I faw in fome broken lavas 

 little empty holes, of the bignefs of a walnut, 

 incruflated all around their inner fides by white or 

 amethyfline femi-pellucid, pointed, or truncated 

 pyramidical cryftallizations, entirely refembling 

 the agate nodules or geodes, which commonly are 

 filled with quartz-cryftallizations. There was no 

 crack or fiffure in the ambient compact lava ; 

 the cryftal fherls pretty hard, and I might rather 

 call them quartz. In the reft of the holes I faw 

 fome fine brownim duft, impalpable and light as 

 allies. This obfervation feems to give a ftrong 

 evidence for the pofiibility of cryftallizations in 

 fire, and to explain the origin of the white garnet- 

 like Iherls in the Italian lavas, which I have men- 

 tioned fo often in my former letters. 



The road from Rome to Oftia is all volcanic 

 ames till within two miles of Oftia ; where I ob- 

 ferved an argillaceous marl, and, for the moft 

 part, fandy ground with inclofed fea-fliells, proving 

 its. having been produced by the alluvions of fea 

 fands. A great number of fea plants, which ap- 

 peared on it, confirmed that opinion. I obferved 

 the following fpecies : Polygonum maritimum f 

 Scirpus maritimus, Vitex agnus caftus, Triglocbin 

 naritiftHtttt) Ciypeola maritime Cbelidonium glan* 

 cium, Tamarix gallic a y Piftacia lent i feus, Myrtus 



comrnu- 



