AMONG THE VOLCANOS AND GLACIERS OF AUVERGNE. 13 « 
greatly disturbed and altered the beds at the point of contact* 
The freshwater beds may be seen tilted at a considerable angle, 
and when in contact with the lava have been altered into a 
hard compact limestone, with conchoidal fracture. At a consi- 
derable distance below the upper basalt of the platform a thick 
bed of consolidated volcanic ash is seen resting upon the 
freshwater limestone ; it has put on the appearance of a lava 
bed, but the freshwater strata below it do not seem in the least 
altered. 
We have not space to indicate half the interesting points to 
visit round Gergovia, but we may advise our brothers of the 
hammer to take an omelet at the little auberge at Merdogne, 
which is believed to be the locality noted by Csesar in his 
account of the attack on Gergovia, according to the plan pub- 
lished in Napoleon III.’s * “ Vie de Cesar.” The hill above 
the village of La Eoche Blanche is recognised as the “ Collis 
sub ipsis radicibus Montis,” which Caesar seized by a night 
attack, and connected with his principal camp by a double foss ; 
and Merdogne is fixed upon as the ground where Caesar sta- 
tioned himself with the 10th Legion, and from whence he 
caused the retreat to be sounded. I also would direct attention 
to the mass of subangular drift with large angular masses of 
rock contained in it, which occurs in a cutting of the road 
between Merdogne and the high road to Clermont. The 
northern shoulder of Montrognon looks as if volcanic masses 
had flowed down the slope since the isolation of that remark- 
able hill from that of Gergovia, with which it must once have 
been continuous. The geology of Montrognon is a good deal 
masked by the fallen ruins of the fortress and the weathering 
of the basalt. 
Another expedition from Clermont for the older basalts is to 
go by rail or drive to Pont du Chateau for the Puy de Dallet. 
Driving, we pass on the right the Puy de la Poix, a local 
outburst of volcanic matter, into the lacustrine marls of the 
ancient lake. A great quantity of bitumen is associated with 
calcareous fragments, volcanic peperino, and vegetable matter. 
The Puy de Crouel and the hill of Clermont are both supposed 
to have the same origin as the Puy de la Poix, viz., the out- 
burst of lava into the waters of the freshwater lake among 
masses of driftwood and other vegetable matter. At Pont du 
Chateau, on our visit in May last, a quarry opened at the end 
of the village before descending to the bridge over the Allier, 
exhibited a section of what was evidently a volcanic vent into 
the freshwater beds, showing that at this point also volcanic 
eruptions were going on during the formation of the lacustrine 
silts at the bottom of the lake. When here some years ago, 
in the month of July, we were walking up the right bank of 
