542 
ELEMENTS OF GEOLOGY. 
an insignificant feature in the great pile, and, like it, may 
frequently have been destroyed and renovated. 
Kespecting the age of the great mass of Mont Dor, we 
can not come at present to any positive decision, because no 
organic remains have yet been found in the tuifs, except im¬ 
pressions of the leaves of trees of species not yet determined. 
It has already been stated (p. 234) that the earliest eruptions 
must have been posterior in origin to those grits and con¬ 
glomerates of the fresh-water formation of the Limagne which 
contain no pebbles of volcanic rocks. But there is evidence 
at a few points, as in the hill of Gergovia, presently to be 
iientioned, that some eruptions took place before the great 
lakes were drained, while others occurred after the desicca¬ 
tion of those lakes, and when deep valleys had already been 
excavated through fresh-water strata. 
The valley in which the cone of Tartaret, above mentioned 
(p. 527), is situated affords an impressive monument of the 
very different dates at which the igneous eruptions of Au¬ 
vergne have happened; for while the cone itself is of Post- 
Pliocene date, the valley is bounded by lofty precipices com¬ 
posed of sheets of ancient columnar trachyte and basalt, 
which once flowed from the summit of Mont Dor in some 
part of the Miocene period. These Miocene lavas had accu¬ 
mulated to a thickness of nearly 1000 feet before the ravine 
was cut down to the level of the river Couze, a river which 
was at length dammed up by the modern cone and the upper 
part of its course transformed into a lake. 
Gergovia. —It has been supposed by some observers that 
there is an alternation of a contemporaneous sheet of lava 
with fresh-water strata in the hill of Gergovia, near Clermont. 
Fig. 604. 
Basaltic capping. 
White and yellow 
marl. 
Blue marls. 
Tuffs. 
Dike. 
White and 
green marls. 
Hill of Gergovia. 
