Geology. 461 



durability, for the architecture of bridges.* . ... Another pur- 

 pose for which the lava of Mennig became quarried was sug- 

 gested by the familiar use which the Romans had made of a 

 similar rock in their own volcanic country, for the construc- 

 tion of portable millstones or hand-mills. ... So soon as it 

 was known that a quarry had been opened in the country of 

 the Ubii, and that a stone might be procured resembling 

 such lavas of Italy as were adapted to the purpose of mill- 

 stones, the demand for it in the Roman stations became 

 general. An intelligent antiquary and naturalist, W. C. Tre- 

 velyan, Esq., of Wallington, has informed me that he pro- 

 cured a portion of a Roman millstone, composed of the 

 lava of Mennig, from the remains of a villa in Northumber- 

 land, near the Hunnum of the ancients. In the Roman 

 station also of Aldborough, in Yorkshire, the Isurium of 

 Ricard, the portable mills which have been found show 

 that the same use was, in other places, made of the lava of 

 Mennig. , ' To which is added in a note, — " My friend, Mr. 

 Trevelyan, whom I have to thank for this curious inform- 

 ation, presented me with a small specimen, broken off from a 

 millstone found near the Hunnum of the Romans, which I 

 am fully persuaded may be referred to the peculiar lava of 

 Mennig." 



I am glad to have an opportunity of adducing an additional 

 testimony to this curious fact. In the autumn of 1822, I 

 found, among the diluvial gravel in the neighbourhood of 

 Godmanchester, in Huntingdonshire, two or three small 

 fragments of porous lava, one side of which appeared to have 

 been ground into a smooth surface. As they were evidently 

 portions of a mass which had been employed in some arti- 

 ficial work, and seemed to have been used in a mill, I made 

 enquiries of different millers, but could not find that such a 

 material was ever employed : I therefore presumed that they 

 were diluvial, and added them to a collection I was then 

 making. 



Since then, I have visited the volcanic district of the Rhine 

 three times ; and in these excursions I have seen the Nieder- 

 mennig lava in situ, as well as at Andernach in the shape of 

 millstones. At the first glance I was convinced that the 



* The lavas, trachytes, and basalts of the Rhone have been extensively 

 used in architecture. Dr. Hibbert quotes the old Roman bridge at Engers, 

 and the present bridge at Treves. I may add, that the cathedral of Cologne 

 is built of trachyte, from the Drachenfels and Wolkenberg in the Sieben- 

 gebirge ; and perhaps that rock cannot be more successfully studied, as to 

 its contents, than in the walls and pillars of that splendid church, for the 

 half polish they have received exhibits the minerals very perfectly. 



