VOL. LXVIII.] PHILOSOPHICAL TRANSACTIONS. 277 



and, by the shape and appearance of the rest of these mountains, doubtless they 

 are all equally composed of the same volcanic substances. The craters on the 

 mountains are discernible, though much altered, and filled up by time and the 

 rubbish thrown from the quarries that are constantly worked on their tops. 

 On each side of the Rhine, most of the way from Bonn to Coblenz, particu- 

 larly between Prohl and Andernach, are high rocks of lava or tuffa. Where the 

 volcanos had not operated, the mountains and rocks are of slate. At Erpel, in 

 a mountain close to the river, and opposite the convent situated on an island 

 about 3 leagues from Bonn, there are some traces of basaltic columns, the 

 quarry seeming to have been nearly exhausted. I have often thought (and this 

 exhausted quarry brings it to my mind again) that the reason why there are 

 scarcely any remains of lavas that have taken the columnal form on Vesuvius and 

 the volcanos near Naples is, that they have been carried off for the use of 

 paving the great Roman roads. The Appian way is mostly composed of lava of 

 a pentagonal and hexagonal form, and seems evidently made of pieces of such 

 basaltic columns. These lavas being ready cut by nature, would naturally be 

 carried off first, as the cutting of solid rocks of lava for such purposes is at- 

 tended with very great expence. 



At Unkel, above a league farther on towards Coblenz, just opposite the town 

 on the other side of the Rhine, is the great quarry belonging to the elector- 

 palatine, which affords a most pleasing and uncommon sight : it is entirely 

 composed of the most regular detached basaltic columns, and though millions of 

 these columns have been extracted, as the towns of Cologne and Bonn testify, 

 yet the quarry is very rich. They lie mostly in an horizontal direction, but some 

 are perpendicular, and others inclining towards the Rhine, which, being very 

 low, shows many of them in the bed of the river itself; they rise from thence 

 into the mountain, where is the present quarry, above 100 feet. They are 

 chiefly pentagonal ; the smallest are in general the most distinct and regular, 

 about 6 inches diameter ; the largest of the columns were about 3 feet long, and 

 about 1-i- foot in diameter. The other lavas in this neighbourhood are of the 

 same substance, and some incline to the same forms, but none so regular. I 

 have not the least doubt but that all basaltes, wherever they exist, have originated 

 subterraneous fire, and are true lavas. 



At Andernach, between Bonn and Coblenz, I saw vast heaps of tuffa ready 

 cut, lying on the banks of the Rhine, and some Dutch vessels loading it ; on 

 inquiry I found, that a considerable trade of this material is carried on between 

 this town and Holland, where they grind down this sort of stone by wind-mills 

 into a powder, which they use as a puzzolane for all their buildings under water. 

 This also corresponds with an idea mentioned in one of my former letters to the 

 R. s., that the tuffas of Naples were composed of a puzzolane, prepared by vol- 



