ose the whole in this
state laid under water at the return of stream tides, and exposed to the
ordinary sedimentary action. Does it not seem probable that the
alternating beds in all their conditions would be given us by such a
process? In the stratum represented by the mud bank, the stone would be
of what I have termed a _felted_, not a fissile character; its organic
remains would exist in a fragmentary and scattered state,--for, torn up
from their places of original deposition, and rolled onwards in the
storm-impelled mud, they could not fail to be broken up and dispersed;
and further, they would be in large part those of bulky deep-sea fishes.
And lastly, the surface of these beds would be polygonally cracked and
flawed, and the wider cracks filled up by the substance of the overlying
strata. And these overlying strata, on the other hand,--the result of
a period of quiet deposition in shallow water,--would be regularly
bedded, and their ichthyic remains, consisting mainly of small littoral
fishes, would be preserved in a state of comparative entireness.
For, however, such numerous repetitions of alternately _felted_
and fissile ripple-marked strata as we find in the neighborhood of
Thurso,--repetitions carried on for hundreds of feet in vertical
extent,--we require yet another condition,--that condition of gradual
subsidence in the general crust which can alone account for the fact so
often pressed upon the geologist in exploring the Coal Measures, that in
deposits thousands of feet in thickness, each stratum in succession had
been laid down in a shallow sea.]

It is a curious circumstance, that the Old Red flagstones which lie
along the southern flanks of the Grampians, and are represented by the
gray stone known in commerce as the Arbroath Pavement, have not, so far
as is yet known, an organism in common with the Old Red flagstones of
the north. I at one time supposed that the rectilinear, smooth-stemmed
fucoid, already described, occurred in both series, as the gray stones
have also their smooth-stemmed, rectilinear, tape-like organism; but the
points of resemblance were too few and simple to justify the conclusion
that they were identical, and I have since ascertained that they were
entirely different plants. The fucoid of the Caithness flagstones threw
off, as I have shown, in the alternate order, numerous ribbon-like
branches or fronds; whereas the ribbon-like fronds or branches of the
Forfarshire plant rose by