Above the yellow beds, or Downton sandstone, as they are called, organic
remains are extensively diffused through the argillaceous strata, which
have yielded fragments of fishes’ bones (being the earliest trace yet
found of vertebrate life), with seeds and land-plants, the latter
clearly indicating the neighbourhood of land, and the poverty of
numbers and the small size of the shells, a change of condition in the
nature of the waters in which they lived. “It was the central part
only,” says Sir R. Murchison, “of this band, or a ginger-bread-coloured
layer of a thickness of three or four inches, and dwindling away to a
quarter of an inch, exhibiting, when my attention was first directed to
it, a matted mass of bony fragments, for the most part of small size and
of very peculiar character. Some of the fragments of fish are of a
mahogany hue, but others of so brilliant a black that when first
discovered they conveyed the impression that the bed was a heap of
broken beetles.”[39]
[39] “Siluria,” p. 148.
[Illustration: Fig. 26.--Halysites catenularius.]
[Illustration: Fig. 27.--Pterygotus bilobatus.]
The fragments thus discovered were, after examination on the spot,
supposed to be those of fishes, but, upon further investigation, many of
them were found to belong to Crustaceans. The ichthyic nature of some of
them is, however, now well established.
[Illustration: Fig. 28.--Plants of the Palæozoic Epoch.--1 and 2, Algæ;
3 and 4, Lycopods.]
Silurian Rocks are found in France in the departments of La Manche,
Calvados, and of the Sarthe, and in Languedoc the Silurian formation has
occupied the attention of Messrs. Graff and Fournet, who have traced
along the base of the Espinouse, the green, primordial chlorite-schists,
surmounted by clay-slates, which become more and more pure as the
distance from the masses of granite and gneiss increases, and the valley
of the Jour is approached. Upon these last the Silurian system rests,
sinking towards the plain under Secondary and Tertiary formations. In
Great Britain, Silurian strata are found enormously developed in the
West and South Highlands of Scotland, on the western slopes of the
Pennine chain and the mountains of Wales, and in the adjoining counties
of Shropshire--their most typical region--and Worcestershire. In Spain;
in Germany (on the banks of the Rhine); in Bohemia--where, also, they
are largely developed, especially in the neighbourhood of Prague--in
Sweden, where they compose the entire island of Gothland; in Norway; in
Russia, especially in the Ural Mountains; and in America, in the
neighbourhood of New York, and half way across the continent--in all
these countries they are more or less developed.
We may add, as a general characteristic of the Silurian system as a
whole, that of all formations it is the most disturbed. In the countries
where it prevails, it only appears as fragments which have escaped
destruction amid the numerous changes that have affected it during the
earlier ages of the world. The beds, originally horizontal, are turned
up, contorted, folded over, and sometimes become even vertical, as in
the slates of Angers, Llanberis, and Ireleth. D’Orbigny found the
Silurian beds with their fossils in the American Andes, at the height of
16,000 feet above the level of the sea. What vast upheavals must have
been necessary to elevate these fossils to such a height!
In the Silurian period the sea still occupied the earth almost entirely;
it covered the greater part of Europe: all the area comprised between
Spain and the Ural was under water. In France only two islands had
emerged from the primordial ocean. One of them was formed of the
granitic rocks of what are now Brittany and La Vendée; the other
constituted the great central plateau, and consisted of the same rocks.
The northern parts of Norway, Sweden, and of Russian Lapland formed a
vast continental surface. In America the emerged lands were more
extensive. In North America an island extended over eighteen degrees of
latitude, in the part now called New Britain. In South America, in the
Pacific, Chili formed one elongated island. Upon the Atlantic, a portion
of Brazil, to the extent of twenty degrees of latitude, was raised above
water. Finally, in the equatorial regions, Guiana formed a later island
in the vast ocean which still covered most other parts of the New World.
There is, perhaps, no scene of greater geological confusion than that
presented by the western flanks of the Pennine chain. A line drawn
longitudinally from about three degrees west of Greenwich, would include
on its western side Cross Fell, in Cumberland, and the greater part of
the Silurian rocks belonging to the Cambrian system, in which the
Cambrian and Lower Silurian rocks are now well determined; while the
upper series are so metamorphosed by eruptive granite and the effects of
denudation, as to be scarcely recognisable. “With the rare exception of
a seaweed and a zoophyte,” says the author of ‘Siluria,’ “not a trace of
a fossil has been detected in the thousands of feet of strata, with
interpolated igneous matter, which intervene between the slates of
Skiddaw and the Coniston limestone, with its overlying flags; at that
zone only do we begin to find anything like a fauna: here, judging from
its fossils, we find representations of the Caradoc and Bala rocks.”
This much-disturbed district Professor Sedgwick, after several years
devoted to its study, has attempted to reconstruct, the following being
a brief summary of his arguments. The region consists of:--
I. Beds of mudstone and sandstone, deposited in an ancient sea,
apparently without the calcareous matter necessary to the existence of
shells and corals, and with numerous traces of organic forms of Silurian
age--these were the elements of the Skiddaw slates.
II. Plutonic rocks were, for many ages, poured out among the aqueous
sedimentary deposits; the beds were broken up and re-cemented--plutonic
silt and other finely comminuted matter were deposited along with the
igneous rocks: the process was again and again repeated, till a deep sea
was filled up with a formation many thousands of feet thick by the
materials forming the middle Cambrian rocks.
III. A period of comparative repose followed. Beds of shells and bands
of coral were formed upon the more ancient rocks, interrupted with beds
of sand and mud; processes many times repeated: and thus, in a long
succession of ages, were the deposits of the upper series completed.
IV. Towards the end of the period, mountain-masses and eruptive rocks
were pushed up through the older deposits. After many revolutions, all
the divisions of the slate-series were upheaved and contorted by
movements which did not affect the newer formations.
V. The conglomerates of the Old Red Sandstone were now spread out by the
beating of an ancient surf, continued through many ages, against the
upheaved and broken slates.
VI. Another period of comparative repose followed: the coral-reefs of
the mountain limestone, and the whole carboniferous series, were formed,
but not without any oscillations between the land and sea-levels.
VII. An age of disruption and violence succeeded, marked by the
discordant position of the rocks, and by the conglomerate of the New Red
Sandstone. At the beginning of this period the great north and south
“Craven fault,” which rent off the eastern calcareous mountains from the
old slates, was formed. Soon afterwards the disruption of the great
“Pennine fault,” which ranges from the foot of Stanmore to the coast of
North Cumberland, occurred, lifting up the terrace of Cross Fell above
the plain of the Eden. About the same time some of the north and south
fissures, which now form the valleys leading into Morecambe Bay, may
have been formed.
VIII. The more tranquil period of the New Red Sandstone now dawns, but
here our facts fail us on the skirts of the Lake Mountains.
IX. Thousands of ages rolled away during the Secondary and Tertiary
periods, in which we can trace no movement. But the powers of Nature are
never still: during this age of apparent repose many a fissure may have
started into an open chasm, many a valley been scooped out upon the
lines of “fault.”
X. Close to the historic times we have evidence of new disruptions and
violence, and of vast changes of level between land and sea. Ancient
valleys probably opened out anew or extended, and fresh ones formed in
the changes of the oceanic level. Cracks among the strata may now have
become open fissures, vertical escarpments formed by unequal elevations
along the lines of fault; and subsidence may have given rise to many of
the tarns and lakes of the district.
Such is the picture which one of our most eminent geologists gives as
the probable process by which this region has attained its present
appearance, after he had devoted years of study and observation to its
peculiarities; and his description of one spot applies in its general
scope to the whole district. At the close of the Silurian period our
island was probably an archipelago, ranging over ten degrees of
latitude, like many of the island groups now found in the great Pacific
Ocean; the old gneissic hills of the western coast of Scotland,
culminating in the granite range of Ben Nevis, and stretching to the
southern Grampians, forming the nucleus of one island group; the south
Highlands of Scotland, ranging from the Lammermoor hills, another; the
Pennine chain and the Malvern hills, the third, and most easterly group;
the Shropshire and Welsh mountains, a fourth; and Devon and Cornwall
stretching far to the south and west. The basis of the calculation
being, that every spot of this island lying now at a lower elevation
than 800 feet above the sea, was under water at the close of the
Silurian period, except in those instances where depression by
subsidence has since occurred.
There is, however, another element to be considered, which cannot be
better stated than in the picturesque language of M. Esquiros, an
eminent French writer, who has given much attention to British geology.
“The Silurian mountains,” he says, “ruins in themselves, contain other
ruins. In the bosom of the Longmynd rocks, geologists discover
conglomerates of rounded stones which bear no resemblance to any rocks
now near them. These stones consequently prove the existence of rocks
more ancient still; they are fragments of other mountains, of other
shores, perhaps even of continents, broken up, destroyed, and crumbled
by earlier seas. There is, then, little hope of one discovering the
origin of life on the globe, since this page of the Genesis of the facts
has been torn. For some years geologists loved to rest their eyes, in
this long night of ages, upon an ideal limit beyond which plants and
animals would begin to appear. Now, this line of demarcation between the
rocks which are without vestiges of organised beings, and those which
contain fossils, is nearly effaced among the surrounding ruins. On the
horizon of the primitive world we see vaguely indicated a series of
other worlds which have altogether disappeared; perhaps it is necessary
to resign ourselves to the fact that the dawn of life is lost in this
silent epoch, where age succeeds age, till they are clothed in the garb
of eternity. The river of creation is like the Nile, which, as Bossuet
says, hides its head--a figure of speech which time has falsified--but
the endless speculations opened up by these and similar considerations
led Lyell to say, ‘Here I am almost prepared to believe in the ancient
existence of the Atlantis of Plato.’”
[Illustration: Fig. 29.--Ischadites Kœnigii. Upper Ludlow Rocks.]
NOTE.--For accurate representations of the typical fossils of
the Palæozoic strata of Britain, the reader may consult, with
advantage, the carefully executed “Figures of Characteristic
British Fossils,” by W. H. Baily, F.G.S. (Van Voorst).