The Student's Elements of Geology Part 54

If many circ.u.mstances favour the theory of the fresh-water origin of the Old Red Sandstone, this view of its nature is not a little confirmed by our finding that it is in Llake Superior and the other inland Canadian seas of fresh water, and in the Mississippi and African rivers, that we at present find those fish which have the nearest affinity to the fossil forms of this ancient formation.

(FIGURE 501. Pterichthys, Aga.s.siz; Upper side, showing mouth; as restored by H.

Miller.)

Among the anomalous forms of Old Red fishes not referable to Huxley's Crossopterygii is the Pterichthys, of which five species have been found in the middle division of the Old Red of Scotland. Some writers have compared their sh.e.l.ly covering to that of Crustaceans, with which, however, they have no real affinity. The wing-like appendages, whence the genus is named, were first supposed by Hugh Miller to be paddles, like those of the turtle; and there can now be no doubt that they do really correspond with the pectoral fins.

The number of species of fish already obtained from the middle division of the Old Red Sandstone in Great Britain is about 70, and the princ.i.p.al genera, besides Osteolepis and Pterichthys, already mentioned, are Glyptolepis, Diplacanthus, Dendrodus, Coccosteus, Cheirancanthus, and Acanthoides.

LOWER OLD RED SANDSTONE.

(FIGURE 502. Cephalaspis Lyellii, Aga.s.siz. Length 6 3/4 inches. From a specimen in my collection found at Glammiss, in Forfarshire. (See other figures, Aga.s.siz, volume 2 table 1 a and 1 b.

a. One of the peculiar scales with which the head is covered when perfect. These scales are generally removed, as in the specimen above figured.

b, c. Scales from different parts of the body and tail.)

The third or lowest division south of the Grampians consists of grey paving- stone and roofing-slate, with a.s.sociated red and grey shales; these strata underlie a dense ma.s.s of conglomerate. In these grey beds several remarkable fish have been found of the genus named by Aga.s.siz Cephalaspis, or "buckler- headed," from the extraordinary shield which covers the head (see Figure 502), and which has often been mistaken for that of a trilobite, such as Asaphus. A species of Pteraspis, of the same family, has also been found by the Reverend Hugh Mitch.e.l.l in beds of corresponding age in Perthshire; and Mr. Powrie enumerates no less than five genera of the family Acanthodidae, the spines, scales, and other remains of which have been detected in the grey flaggy sandstones. (Powrie Geological Quarterly Journal volume 20 page 417.)

(FIGURE 503. Pterygotus anglicus, Aga.s.siz. Middle portion of the back of the head called the seraphim.)

(FIGURE 504. Pterygotus anglicus, Aga.s.siz. Forfarshire. Ventral aspect. Restored by H. Wodward, F.G.S.

a. Carapace, showing the large sessile eyes at the anterior angles.

b. The metastoma or post-oral plate (serving the office of a lower lip).

c, c. Chelate appendages (antennules).

d. First pair of simple palpi (antennae).

e. Second pair of simple palpi (mandibles).

f. Third pair of simple palpi (first maxillae).

g. Pair of swimming feet with their broad basal joints, whose serrated edges serve the office of maxillae.

h. Thoracic plate covering the first two thoracic segments, which are indicated by the figures 1, 2, and a dotted line.

1-6. Thoracic segments.

7-12. Abdominal segments.

13. Telson, or tail-plate.)

In the same formation at Carmylie, in Forfarshire, commonly known as the Arbroath paving-stone, fragments of a huge crustacean have been met with from time to time. They are called by the Scotch quarrymen the "Seraphim," from the wing-like form and feather-like ornament of the thoracic appendage, the part most usually met with. Aga.s.siz, having previously referred some of these fragments to the cla.s.s of fishes, was the first to recognise their crustacean character, and, although at the time unable correctly to determine the true relation of the several parts, he figured the portions on which he founded his opinion, in the first plate of his "Poissons Fossiles du Vieux Gres Rouge."

A restoration in correct proportion to the size of the fragments of P. anglicus (Figure 504), from the Lower Old Red Sandstone of Perthshire and Forfarshire, would give us a creature measuring from five to six feet in length, and more than one foot across.

The largest crustaceans living at the present day are the Inachus Kaempferi, of De Haan, from j.a.pan (a brachyurous or short-tailed crab), chiefly remarkable for the extraordinary length of its limbs; the fore-arm measuring four feet in length, and the others in proportion, so that it covers about 25 square feet of ground; and the Limulus Molucca.n.u.s, the great King Crab of China and the Eastern seas, which, when adult, measures 1 1/2 foot across its carapace, and is three feet in length.

(FIGURE 505. Parka decipiens, Fleming. In sandstone of lower beds of Old Red, Ley's Mill, Forfarshire.)

(FIGURE 506. Parka decipiens, Fleming. In shale of Lower Old Red, Park Hill, Fife.)

(FIGURE 507. Shale of Old Red Sandstone. Forfarshire. With impression of plants and eggs of Crustaceans.

a. Two pair of ova? resembling those of large Salamanders or Tritons-- on the same leaf.

b, b. Detached ova.)

Besides some species of Pterygotus, several of the allied genus Eurypterus occur in the Lower Old Red Sandstone, and with them the remains of gra.s.s-like plants so abundant in Forfarshire and Kincardineshire as to be useful to the geologist by enabling him to identify the inferior strata at distant points. Some botanists have suggested that these plants may be of the family Fluviales, and of fresh-water genera. They are accompanied by fossils, called "berries" by the quarrymen, which they compared to a compressed blackberry (see Figures 505, 506), and which were called "Parka" by Dr. Fleming. They are now considered by Mr. Powrie to be the eggs of crustaceans, which is highly probable, for they have not only been found with Pterygotus anglicus in Forfarshire and Perthshire, but also in the Upper Silurian strata of England, in which species of the same genus, Pterygotus, occur.

The grandest exhibitions, says Sir R. Murchison, of the Old Red Sandstone in England and Wales appear in the escarpments of the Black Mountains and in the Fans of Brecon and Carmarthen, the one 2862, and the other 2590 feet above the sea. The ma.s.s of red and brown sandstone in these mountains is estimated at not less than 10,000 feet, clearly intercalated between the Carboniferous and Silurian strata. No sh.e.l.ls or corals have ever been found in the whole series, not even where the beds are calcareous, forming irregular courses of concretionary lumps called "corn-stones," which may be described as mottled red and green earthy limestones. The fishes of this lowest English Old Red are Cephalaspis and Pteraspis, specifically different from species of the same genera which occur in the uppermost Ludlow or Silurian tilestones. Crustaceans also of the genus Eurypterus are met with.

MARINE OR DEVONIAN TYPE.

We may now speak of the marine type of the British strata intermediate between the Carboniferous and Silurian, in treating of which we shall find it much more easy to identify the Upper, Middle, and Lower divisions with strata of the same age in other countries. It was not until the year 1836 that Sir R. Murchison and Professor Sedgwick discovered that the culmiferous or anthracitic shales and sandstones of North Devon, several thousand feet thick, belonged to the coal, and that the beds below them, which are of still greater thickness, and which, like the carboniferous strata, had been confounded under the general name "graywacke," occupied a geological position corresponding to that of the Old Red Sandstone already described. In this reform they were aided by a suggestion of Mr. Lonsdale, who, after studying the Devonshire fossils, perceived that they belonged to a peculiar palaeontological type of intermediate character between the Carboniferous and Silurian.

It is in the north of Devon that these formations may best be studied, where they have been divided into an Upper, Middle, and Lower Group, and where, although much contorted and folded, they have for the most part escaped being altered by intrusive trap-rocks and by granite, which in Dartmoor and the more southern parts of the same county have often reduced them to a crystalline or metamorphic state.

TABLE 25.1 DEVONIAN SERIES IN NORTH DEVON.

UPPER DEVONIAN OR PILTON GROUP.

a. Sandy slates and schists with fossils, 36 species out of 110 common to the Carboniferous group (Pilton, Barnstaple, etc.), resting on soft schists in which fossils are very abundant (Croyde, etc.), and which pa.s.s down into

b. Yellow, brown, and red sandstone, with land plants (Cyclopteris, etc.) and marine sh.e.l.ls. One zone, characterised by the abundance of cucullaea (Baggy Point, Marwood, Sloly, etc.) resting on hard grey and reddish sandstone and micaceous flags, no fossils yet found (Dulverton, Pickwell, Down, etc.)

MIDDLE DEVONIAN OR ILFRACOMBE GROUP.

a. Green glossy slates of considerable thickness, no fossils yet recorded from these beds (Mortenoe, Lee Bay, etc.).

b. Slates and schists, with several irregular courses of limestone containing sh.e.l.ls and corals like those of the Plymouth Limestone (Combe Martin, Ilfracombe, etc.).

LOWER DEVONIAN OR LYNTON GROUP.

a. Hard, greenish, red, and purple sandstone-- no fossils yet found (Hangman Hill, etc.).

b. Soft slates with subordinate sandstones-- fossils numerous at various horizons-- Orthis, Corals, Encrinites, etc. (Valley of Rocks, Lynmouth, etc.).

Table 25.1 exhibits the sequence of the strata or subdivisions as seen both on the sea-coast of the British Channel and in the interior of Devon. It will be seen that in all main points it agrees with the table drawn up in 1864 for the sixth edition of my "Elements." Mr. Etheridge has since published an excellent account of the different subdivisions of the rocks and their fossils, and has also pointed out their relation to the corresponding marine strata of the Continent. (Quarterly Geological Journal volume 23 1867.) The slight modifications introduced in my table since 1864 are the result of a tour made in 1870 in company with Mr. T. Mck. Hughes, when we had the advantage of Mr.

Etheridge's memoir as our guide.

The place of the sandstones of the Foreland is not yet clearly made out, as they are cut off by a great fault and disturbance.

UPPER DEVONIAN ROCKS.

(FIGURE 508. Spirifera disjuncta, Sowerby. Syn. Sp. Verneuilii, Murch. Upper Devonian, Boulogne.)

(FIGURE 509. Phacops latifrons, Bronn. Characteristic of the Devonian in Europe, Asia, and N. and S. America.)

(FIGURE 510. Clymenia linearis, Munster. Petherwyn, Cornwall; Elbersreuth, Bavaria.)

(FIGURE 511. Cypridina serrato-striata, Sandberger, Weilburg, etc.; Cornwall, Na.s.sau, Saxony, Belgium.)

The slates and sandstones of Barnstaple (a and b of the preceding section) contain the sh.e.l.l Spirifera disjuncta, Sowerby (S. Verneuilii, Murch.), (see Figure 508), which has a very wide range in Europe, Asia Minor, and even China; also Strophalosia caperata, together with the large trilobite Phacops latifrons, Bronn. (See Figure 509), which is all but world-wide in its distribution. The fossils are numerous, and comprise about 150 species of mollusca, a fifth of which pa.s.s up into the overlying Carboniferous rocks. To this Upper Devonian belong a series of limestones and slates well developed at Petherwyn, in Cornwall, where they have yielded 75 species of fossils. The genus of Cephalopoda called Clymenia (Figure 510) is represented by no less than eleven species, and strata occupying the same position in Germany are called Clymenien- Kalk, or sometimes Cypridinen-Schiefer, on account of the number of minute bivalve sh.e.l.ls of the crustacean called Cypridina serrato-striata (Figure 511), which is found in these beds, in the Rhenish provinces, the Harz, Saxony, and Silesia, as well as in Cornwall and Belgium.

MIDDLE DEVONIAN ROCKS.

(FIGURE 512. Heliolites porosa, Goldf. sp. (Porites pyriformis, Lonsd.) a. Portion of the same magnified. Middle Devonian, Torquay, Plymouth; Eifel.)

(FIGURE 513. Favosites cervicornis, Blainv. S. Devon, from a polished specimen.

a. Portion of the same magnified, to show the pores.)