Ch.XIX.] 
VOLCANIC ROCKS— AUVERGNE. 
259 
The bottom of the hill consists of slightly inclined beds of 
white and greenish marls, more than three hundred feet in 
thickness, which are intersected by a dike of basalt, which may 
be studied in the ravine above the village of Merdos-ne. The 
dike here cuts through the marly strata at a considerable angle, 
No. 60. 
Hill of Gergovia. 
producing, in general, great alteration and confusion in them for 
some distance from the point of contact. Above the white and 
green marls, a series of beds of limestone and marl, containing 
fresh-water shells, are seen to alternate with volcanic tuff. In 
the lowest part of this division, beds of pure marl alternate 
with compact fissile tuff resembling some of the subaqueous 
tuffs of Italy and Sicily called peperinos. Occasionally frag- 
ments of scoriae are visible in this rock. Still higher is seen, 
another group of some thickness, consisting exclusively of tuff, 
upon which lie other marly strata intermixed with volcanic 
matter. 
There are many points in Auvergne where igneous rocks 
have been forced by subsequent injection through clays and 
marly limestones, in such a manner that the whole has become 
blended in one confused and brecciated mass, between which 
and the basalt there is sometimes no very distinct line of de- 
marcation. In the cavities of such mixed rocks we often find 
calcedony and crystals of mesotype, stilbite and arragonite. To 
S 2 
