286 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb, 6, 



is built*. The still higher plateau, south of the town, is occupied by 

 great thicknesses of the well-known yellowish tufa of the Campagna, 

 fig. 1 dy in the puzzuolana o? sandy beds of which many caverns have 

 been excavated. Now, if the " tephrine lava '* had happened to con- 

 tain zeolite instead of leucite, any British geologist might call it a 

 trappean greenstone ; and, as it is, there can be no sort of doubt that 

 these Italian earlier volcanic products are as much of subaqueous 

 origin as the blue shelly marine marl and sands which they have 

 traversed, and which they immediately overlie. 



In descending from St. Lorenzo to the Lake of Bolsena, the hard 

 leucite rock is seen to contain crystals of pyroxene, hornblende, 

 felspar, and mica. Occasional fragments of secondary Apennine 

 limestone also occur in this rock, but leucite is the dominant mineral. 

 This leucitic rock is encased in a vast thickness of tufaceous, sandy 

 or marly tufa, in parts a peperino, in parts scoriaceous. It is in 

 this feature of being more scoriaceous, and in here and there having 

 a rough or trachytic aspect, that these eruptive rocks, whether ba- 

 saltic and compact or loose and scoriaceous, more resemble modern 

 lavas and their accompaniments than our British traps. But I ob- 

 served nothing like a regular qua-qua-versal or outward dip from 

 the Lake of Bolsena. The Marquis Pareto, however, who examined 

 the whole circumference, speaks of a slight outward inclination ; but 

 even if it be so, on those sides of the lake which I did not examine, 

 there is little or no analogy between such an arrangement and the 

 highly inclined dejections of a subaerial volcano. Pareto' s words 

 are only to this effect : " Questi banchi inchinano generalmente piu 

 al di fuori, che verso il centra della cavita medesima." The hard, 

 compact leucitic rock, which occurs near the summit of the escarp- 

 ment at the north end of the lake, is again found very little above 

 its level to the south side of the town of Bolsena, where it ranges 

 in devious columnar masses, not unlike many of our prismatic 

 basalts, and is composed of numerous pentagonal and hexagonal 

 columns, the ends of which vary to different angles with the hori- 

 zon, depending probably upon the outline of the cooling surfaces 

 of the pre-existing materials with which the eruptive lava came in 

 contact. Further southward, i. e. on the east bank of the lake, 

 whitish claystones, in parts having a semi-pumiceous or trass-like 

 aspect, alternate with courses of grey-coloured scoriaceous lava. Few 

 persons, who have examined this country, will, I apprehend, be dis- 

 posed to doubt, that the trachytes of Tuscany and the Papal States 

 are among the oldest of these earlier volcanic rocks. Of this, in- 



* In Brongniart's classification of rocks, the basalt of Radicofani is called " Te- 

 phrine pavimenteuse," and is associated both with the lava of Volvic, in Auvergne, 

 and with a production of Monte Nuovo in modern times. And yet nothing can be 

 geologically more distinct than the periods of eruption of these three rocks. Again, 

 the basalt under Acquapendente, which geologically, in my opinion, is simply a 

 varied mineral prolongation of a band issuing from the same centre as tliat of Ra- 

 dicofani, is distinguished as " Tephrine amphigenique," merely because it happens 

 to contain imbedded crystals of amphigene or leucite. — Classification des Roches, 

 pp. 118-119. 



