ne direction has not been perpetual throughout all past
time. There have been great oscillations of level by which a surface of
dry land has been submerged to a depth of several thousand feet, and
then at a period long subsequent raised again and made to emerge. Nor
have the regions now motionless been always at rest; and some of those
which are at present the theatres of reiterated earthquakes have
formerly enjoyed a long continuance of tranquillity. But although
disturbances have ceased after having long prevailed, or have
recommenced after a suspension for ages, there has been no universal
disruption of the earth's crust or desolation of the surface since
times the most remote. The non-occurrence of such a general convulsion
is proved by the perfect horizontally now retained by some of the most
ancient fossiliferous strata throughout wide areas.

_Inferences derived from unconformable strata._--That the subterranean
forces have visited different parts of the globe at successive periods,
is inferred chiefly from the unconformability of strata belonging to
groups of different ages. Thus, for example, on the borders of Wales and
Shropshire we find the slaty beds of the ancient Silurian system curved
and vertical, while the beds of the overlying carboniferous shale and
sandstone are horizontal. All are agreed, that in such a case the older
set of strata had suffered great dislocation before the deposition of
the newer or carboniferous beds, and that these last have never since
been convulsed by any movements of excessive violence. But the strata of
the inferior group suffered only a local derangement, and rocks of the
same age are by no means found everywhere in a curved or vertical
position. In various parts of Europe, and particularly near Lake Wener
in the south of Sweden, and in many parts of Russia, beds of the same
Silurian system maintain the most perfect horizontality; and a similar
observation may be made respecting limestones and shales of the like
antiquity in the great lake district of Canada and the United States.
They are still as flat and horizontal as when first formed; yet since
their origin not only have most of the actual mountain-chains been
uplifted, but the very rocks of which those mountains are composed
have been formed.

It would be easy to multiply instances of similar unconformability in
formations of other ages; but a few more will suffice. The coal measures
before alluded to as horizontal on