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that the volcanic tuffa, basalt, and trachyte,
as well as the scoriaceous and cellular lava,
are in some situations intermixed, or appear
to pass by gradation into each other. Thus
the cellular lava is seen to become less
porous, and terminate in compact basalt,
and the trachyte or semivitrified granite,
contains imbedded masses of scoriæ.
The fresh-water limestone resembles a
coarse chalk, or more properly a chalk
marl : it is in some parts associated with
clay and sandstone. At Gergovia it forms
very regular strata, from two to three feet in
thickness, separated by beds of indurated
argillaceous stone, or what is vulgarly called
clunch : all these beds are nearly horizontal.
In two of the beds of this limestone
there are numerous bones of mammalia,
similar in preservation and appearance
to those of the paléothéria, and other
animals found in the gypsum of Montmartre.
I was not so fortunate as to procure
any of the maxillary bones or teeth,
but I obtained portions of tibia and ribs.
The occurrence of bones of this description
in the limestone of Gergovia, has not,
I believe, been noticed by any geologist,
and it was unknown to the professors at
Paris, when I passed through that city in
1822. The specimens I collected at Gergovia
were packed up and forwarded to
England, or I should have availed myself
of the opportunity of identifying them with
the fossil remains from Montmartre, during
my residence in Paris. The fresh-water
limestone is extensively spread over many
parts of the south of France; and shells
and bones, similar to those in the Paris
basin, have been found in it. The reason
why bones of the mammalia have not been
discovered earlier at Gergovia, is owing to
the recent opening of the quarries where
they occur. The men informed me they
had found one thigh bone, a few months
before I was there, as large as that o f an
ox. I f the fresh-water limestone really
extends under the older volcanic rocks, the
relative age of the latter is fixed as posterior
to the gypsum formation in the Paris
basin.
One of our first excursions from Clermont
was to visit the Puy de Pariou, about
§even miles west of that city. We started
early in the morning. The road ascends
in a zigzag course to the elevated plain, on
which most of the volcanic mountains are
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