VOLCANIC ROCKS OF AUVERGNE. 
541 
Miocene period, but to have been most active during the 
Upper Miocene and Pliocene eras. I have already al¬ 
luded to the grand succession of events of which there is 
evidence in Auvergne since the last retreat of the sea (see 
p. 527). 
The earliest monuments of the Tertiary Period in that re¬ 
gion are lacustrine dejDOsits of great thickness, in the lowest 
conglomerates of which are rounded pebbles of quartz, mica- 
schist, granite, and other non-volcanic rocks, without the 
slightest intermixture of igneous products. To these con¬ 
glomerates succeed argillaceous and calcareous marls and 
limestones, containing Lower Miocene shells and bones of 
mammalia, the higher beds of which sometimes alternate 
with volcanic tulF of contemporaneous origin. After the fill¬ 
ing up or drainage of the ancient lakes, huge piles of trachyt- 
ic and basaltic rocks, with volcanic breccias, accumulated to 
a thickness of several thousand feet, and were superimposed 
upon granite, or the contiguous lacustrine strata. The great¬ 
er portion of these igneous rocks appear to have originated 
during the Upper Miocene and Pliocene periods; and extinct 
quadrupeds of those eras, belonging to the genera Mastodon, 
Rhinoceros, and others, were buried in ashes and beds of al¬ 
luvial sand and gravel, which owe their preservation to over¬ 
spreading sheets of lava. 
In Auvergne, the most ancient and conspicuous of the vol¬ 
canic masses is Mount Dor, which rests immediately on the 
granitic rocks standing apart from the fresh-water strata. 
This great mountain rises suddenly to the height of several 
thousand feet above the surrounding platform, and retains 
the shape of a flattened and somewhat irregular cone, the 
slope of which is gradually lost in the high plain around. 
This cone is composed of layers of scoriae, pumice-stones, and 
their fine detritus, with interposed beds of trachyte and 
basalt, which descend often in uninterrupted sheets until 
they reach and spread themselves round the base of the 
mountain.* Conglomerates, also, composed of angular and 
rounded fragments of igneous rocks, are observed to alternate 
with the above ; and the various masses are seen to dip off 
from the central axis, and to lie parallel to the sloping flanks 
of the mountain. The summit of Mont Dor terminates in 
seven or eight rocky peaks, where no regular crater can now 
be traced, but where we may easily imagine one to have ex¬ 
isted, which may have been shattered by earthquakes, and 
have suffered degradation by aqueous agents. Originally, 
perhaps, like the highest crater of Etna, it may have formed 
* Scrope’s Central France, p. 98. 
