I
uà
'7‘i
i ’i
f
J -
Near the middle of the hill there is a ridge
of stone of the same kind, but harder than
the rest, which cuts across the hill from
east to west, rising above the surface and
dipping down to the north, as in the annexed
figure. N. S. represent the north
and south points of the summit, and A.
the ridge which appears to intersect the hill
like a dyke. Between the southern end S.
and the ridge or dyke, there is a considerable
depression.
I am not aware that the arrangement of
the strata in this hill has been before
noticed, but it is very distinctly marked,
and is a circumstance of some importance,
as it proves that the position of
the beds in the Puy de Cruelle has undergone
a great change since their original deposition
; and if this be admitted, we cannot
hesitate in admitting also that similar
changes may have taken place, in other
volcanic mountains in this district, whose
present position has greatly perplexed geologists.
The appearance of stratification is
not uncommon in volcanic rocks, and may
sometimes arise from the change of temperature
in cooling, producing regular fissures
resembling strata ; but the tuffaceous
4 ■' I.!
or brecciated matter of the Puy de Cruelle,
was probably deposited by an aqueous
eruption of different layers, covering each
other in succession at short intervals. The
dip of the beds towards the centre of the
hill, would admit of a natural explanation
by a sudden subsidence of the ground in
that part, a subsidence which might well
take place in a country rendered cavernous
by the ejection of so much volcanic matter.
But a more important enquiry presents
itself. I f the Puy de Cruelle be the remains
of a larger bed of volcanic tuffa, once
connected with the tuffa in mountains to
the west, has it been detached by the
erosion of w a te r; or by some elevating
force acting from beneath ; or by the subsidence
of the surrounding mass, which
has left this hill, with its strata, much shattered,
but still retaining their original
elevation ? Dr.Daubenny, in the fourth volume
of the Edinburgh Phil. Journal, states,
that the limestone on which this rock appears
to rest, sends up processes into the
volcanic matter, and that isolated masses
of limestone are found in the volcanic rock.
I did not observe these processes, as my
attention was more occupied with the ,