528 
ELEMENTS OF GEOLOGY. 
lapicida^ 2 iXidi Clausilia riigosa. That the current which has 
issued from the Puy de Tartaret may, nevertheless, be very 
ancient in reference to the events of human history, we may 
conclude, not only from the divergence of the mammiferous 
fauna from that of our day, but from the fact that a Roman 
bridge of such form and construction as continued in use 
only down to the fifth century, but which may be older, is 
now seen at a place about a mile and a half from St. Kec- 
taire. This ancient bridge spans the river Couze with two 
arches, each about fourteen feet wide. These arches spring 
from the lava of Tartaret, on both banks, showing that a 
ravine precisely like that now existing had already been ex¬ 
cavated by the river through that lava thirteen or fourteen 
centuries ago. 
While the river Couze has in most cases, as at the site of 
this ancient bridge, been simply able to cut a deep channel 
through the lava, the lower portion of which is shown to be 
columnar, the same torrent has in other places, where the 
valley was contracted to a narrow gorge, had power to re¬ 
move the entire mass of basaltic rock, causing for a short 
space a complete breach of continuity in the volcanic cur¬ 
rent. The work of erosion has been very slow, as the basalt 
is tough and hard, and one column after another must have 
been undermined and reduced to pebbles, and then to sand. 
During the time required for this operation, the perishable 
cone of Tartaret, occupying the lowest part of the great val¬ 
ley descending from Mont Dor (see p. 542), and damming up 
the river so as to cause the Lake of Chambon, has stood unin¬ 
jured, proving that no great flood or deluge can have passed 
over this region in the interval between the eruption of Tar¬ 
taret and our own times. 
Piiy de Come ,—The Puy de Come and its lava-current, near 
Clermont, may be mentioned as another minor volcano of 
about the same age. This conical hill rises from the gran¬ 
itic platform, at an angle of between 30° and 40°, to the 
height of more than 900 feet. Its summit presents two dis¬ 
tinct craters, one of them with a vertical depth of 250 feet. 
A stream of lava takes its rise at the western base of the hill 
instead of issuing from either crater, and descends the granit¬ 
ic slope towards the present site of the town of Pont Gibaud. 
Thence it pours in a broad sheet down a steep declivity into 
the valley of the Sioule, filling the ancient river-channel for 
the distance of more than a mile. The Sioule, thus dispos¬ 
sessed of its bed, has worked out a fresh one between the 
lava and the granite of its western bank; and the excava- 
