The Student's Elements of Geology Part 58

(FIGURE 545. Graptolithus priodon, Bronn. Ludlow and Wenlock shales.)

The Wenlock Shale, observes Sir R. Murchison, is infinitely the largest and most persistent member of the Wenlock formation, for the limestone often thins out and disappears. The shale, like the Lower Ludlow, often contains elliptical concretions of impure earthy limestone. In the Malvern district it is a ma.s.s of finely levigated argillaceous matter, attaining, according to Professor Phillips, a thickness of 640 feet, but it is sometimes more than 1000 feet thick in Wales, and is worked for flag-stones and slates. The prevailing fossils, besides corals and trilobites, and some crinoids, are several small species of Orthis, Cardiola, and numerous thin-sh.e.l.led species of Orthocerat.i.tes.

About six species of Graptolite, a peculiar group of sertularian fossils before alluded to as being confined to Silurian rocks, occur in this shale. Of fossils of this genus, which is very characteristic of the Lower Silurian, I shall again speak in the sequel.

b. WOOLHOPE BEDS.

Though not always recognised as a separate subdivision of the Wenlock, the Woolhope beds, which underlie the Wenlock shale, are of great importance.

Usually they occur as ma.s.sive or nodular limestones, underlaid by a fine shale or flag-stone; and in other cases, as in the noted Denbighs.h.i.+re sandstones, as a coa.r.s.e grit of very great thickness. This grit forms mountain ranges through North and South Wales, and is generally marked by the great sterility of the soil where it occurs. It contains the usual Wenlock fossils, but with the addition of some common in the uppermost Ludlow rock, such as Chonetes lata and Bellerophon trilobatus. The chief fossils of the Woolhope limestone are Illaenus Barriensis, Homalonotus delphinocephalus (Figure 544), Strophomena imbrex, and Rhynchonella Wilsoni (Figure 531). The latter attains in the Woolhope beds an unusual size for the species, the specimens being sometimes twice as large as those found in the Wenlock limestone.

In some places below the Wenlock formation there are shales of a pale or purple colour, which near Tarannon attain a thickness of about 1000 feet; they can be traced through Radnor and Montgomery to North Wales, according to Messrs. Jukes and Aveline. By the latter geologist they have been identified with certain shales above the May-Hill Sandstone, near Llandovery, but, owing to the extreme scarcity of fossils, their exact position remains doubtful.

3. LLANDOVERY GROUP-- BEDS OF Pa.s.sAGE.

We now come to beds respecting the cla.s.sification of which there has been much difference of opinion, and which in fact must be considered as beds of pa.s.sage between Upper and Lower Silurian. I formerly adopted the plan of those who cla.s.s them as Middle Silurian, but they are scarcely ent.i.tled to this distinction, since after about 1400 Silurian species have been compared the number peculiar to the group in question only gives them an importance equal to such minor subdivisions as the Ludlow or Bala groups. I therefore prefer to regard them as the base of the Upper Silurian, to which group they are linked by more than twice as many species as to the Lower Silurian. By this arrangement the line of demarkation between the two great divisions, though confessedly arbitrary, is less so than by any other. They are called Llandovery Rocks, from a town in South Wales, in the neighbourhood of which they are well developed, and where, especially at a hill called Noeth Grug, in spite of several faults, their relations to one another can be clearly seen.

a. UPPER LLANDOVERY OR MAY-HILL SANDSTONE.

(FIGURE 546. Pentamerus oblongus, Sowerby. Upper and Lower Llandovery beds.

a, b. Views of the sh.e.l.l itself, from figures in Murchison's Sil. Syst.

c. Cast with portion of sh.e.l.l remaining, and with the hollow of the central septum filled with spar.

d. Internal cast of a valve, the s.p.a.ce once occupied by the septum being represented by a hollow in which is seen a cast of the chamber within the septum.)

(FIGURE 547. Stricklandinia (Pentamerus) lirata, Sowerby.)

The May-Hill group, which has also been named "Upper Llandovery," by Sir R.

Murchison, ranges from the west of the Longmynd to Builth, Llandovery, and Llandeilo, and to the sea in Marlow's Bay, where it is seen in the cliffs. It consists of brownish and yellow sandstones with calcareous nodules, having sometimes a conglomerate at the base derived from the waste of the Lower Silurian rocks. These May-Hill beds were formerly supposed to be part of the Caradoc formation, but their true position was determined by Professor Sedgwick to be at the base of the Upper Silurian proper. (Quarterly Geological Journal volume 4 page 215 1853.) The more calcareous portions of the rock have been called the Pentamerus limestone, because Pentamerus oblongus (Figure 546) is very abundant in them. It is usually accompanied by P. (Stricklandinia) lirata (Figure 547); both forms have a wide geographical range, being also met with in the same part of the Silurian series in Russia and the United States.

About 228 species of fossils are known in the May-Hill division, more than half of which are Wenlock species. They consist of trilobites of the genera Illaenus and Calymene; Brachiopods of the genera Orthis, Atrypa, Leptaena, Pentamerus, Strophomena, and others; Gasteropods of the genera Turbo, Murchisonia (for genus, see Figure 567), and Bellerophon; and Pteropods of the genus Conularia.

The Brachiopods, of which there are 66 species, are almost all Upper Silurian.

(FIGURE 548. Tentaculites annulatus, Schlot. Interior casts in sandstone. Upper Llandovery, Eastnor Park, near Malvern. Natural size and magnified.)

Among the fossils of the May-Hill sh.e.l.ly sandstone at Malvern, Tentaculites annulatus (Figure 548), an annelid, probably allied to Serpula, is found.

LOWER LLANDOVERY ROCKS.

Below the May-Hill Group are the Lower Llandovery Rocks, which consist chiefly of hard slaty rocks, and beds of conglomerate from 600 to 1000 feet in thickness. The fossils, which are somewhat rare in the lower beds, consist of 128 known species, only eleven of which are peculiar, 83 being common to the May-Hill group above, and 93 common to the rocks below. Stricklandinia (Pentamerus) levis, which is common in the Lower Llandovery, becomes rare in the Upper, while Pentamerus oblongus (Figure 546), which is the characteristic sh.e.l.l of the Upper Llandovery, occurs but seldom in the Lower.

LOWER SILURIAN ROCKS.

The Lower Silurian has been divided into, first, the Bala Group; secondly, the Llandeilo flags; and, thirdly, the Arenig or Lower Llandeilo formation.

BALA AND CARADOC BEDS.

(FIGURE 549. Orthis tricenaria, Conrad. New York; Canada. 1/2 natural size.)

(FIGURE 550. Orthis vespertilio, Sowerby. Shrops.h.i.+re, N. and S. Wales. One-half natural size.)

(FIGURE 551. Orthis (Strophomena) grandis, Sowerby. Two-thirds natural size.

Caradoc Beds, Horderley, Shrops.h.i.+re, and Coniston, Lancas.h.i.+re.)

The Caradoc sandstone was originally so named by Sir R.I. Murchison from the mountain called Caer Caradoc, in Shrops.h.i.+re; it consists of sh.e.l.ly sandstones of great thickness, and sometimes containing much calcareous matter. The rock is frequently laden with the beautiful trilobite called by Murchison Trinucleus Caractaci (see Figure 553), which ranges from the base to the summit of the formation, usually accompanied by Strophomena grandis (see Figure 551), and Orthis vespertilio (Figure 550), with many other fossils.

BRACHIOPODA.

Nothing is more remarkable in these beds, and in the Silurian strata generally of all countries, than the preponderance of brachiopoda over other forms of mollusca. Their proportional numbers can by no means be explained by supposing them to have inhabited seas of great depth, for the contrast between the palaeozoic and the present state of things has not been essentially altered by the late discoveries made in our deep-sea dredgings. We find the living brachiopoda so rare as to form about one forty-fourth of the whole bivalve fauna, whereas in the Lower Silurian rocks of which we are now about to treat, and where the brachiopoda reach their maximum, they are represented by more than twice as many species as the Lamellibranchiate bivalves.

There may, indeed, be said to be a continued decrease of the proportional number of this lower tribe of mollusca as we proceed from older to newer rocks. In the British Devonian, for example, the Brachiopoda number 99, the Lamellibranchiata 58; while in the Carboniferous their proportions are more than reversed, the Lamellibranchiata numbering 334 species, and the Brachiopoda only 157. In the Secondary or Cainozoic formations the preponderance of the higher grade of bivalves becomes more and more marked, till in the tertiary strata it approaches that observed in the living creation.

While on this subject, it may be useful to the student to know that a Brachiopod differs from ordinary bivalves, mussels, c.o.c.kles, etc., in being always equal- sided and never quite equi-valved; the form of each valve being symmetrical, it may be divided into two equal parts by a line drawn from the apex to the centre of the margin.

TRILOBITES.

In the Bala and Caradoc beds the trilobites reach their maximum, being represented by 111 species referred to 23 genera.

(FIGURE 552. Young individuals of Trinucleus concentricus (T. ornatus, Barr.).

a. Youngest state. Natural size and magnified; the body rings not at all developed.

b. A little older. One thorax joint.

c. Still more advanced. Three thorax joints. The fourth, fifth, and sixth segments are successively produced, probably each time the animal moulted its crust.)

(FIGURE 553. Trinucleus concentricus, Eaton. Syn. T. Caractaci, Murch. Ireland; Wales; Shrops.h.i.+re; North America; Bohemia.)

Burmeister, in his work on the organisation of trilobites, supposes that they swam at the surface of the water in the open sea and near coasts, feeding on smaller marine animals, and to have had the power of rolling themselves into a ball as a defence against injury. He was also of opinion that they underwent various transformations a.n.a.logous to those of living crustaceans. M. Barrande, author of an admirable work on the Silurian rocks of Bohemia, confirms the doctrine of their metamorphosis, having traced more than twenty species through different stages of growth from the young state just after its escape from the egg to the adult form. He has followed some of them from a point in which they show no eyes, no joints, or body rings, and no distinct tail, up to the complete form with the full number of segments. This change is brought about before the animal has attained a tenth part of its full dimensions, and hence such minute and delicate specimens are rarely met with. Some of his figures of the metamorphoses of the common Trinucleus are copied in Figures 552 and 553. It was not till 1870 that Mr. Billings was enabled, by means of a specimen found in Canada, to prove that the trilobite was provided with eight legs.

(FIGURE 554. Palaeaster asperimus, Salt. Caradoc, Welshpool.)

(FIGURE 555. Echinosphaeronites balticus, Eichwald. (Of the family Cystideae.) a. Mouth.

b. Point of attachment of stem. Lower Silurian S. and N. Wales.)

It has been ascertained that a great thickness of slaty and crystalline rocks of South Wales, as well as those of Snowdon and Bala, in North Wales, which were first supposed to be of older date than the Silurian sandstones and mudstones of Shrops.h.i.+re, are in fact identical in age, and contain the same organic remains.

At Bala, in Merioneths.h.i.+re, a limestone rich in fossils occurs, in which two genera of star-fish, Protaster and Palaeaster, are found; the fossil specimen of the latter (Figure 554) being almost as uncompressed as if found just washed up on the sea-beach. Besides the star-fish there occur abundance of those peculiar bodies called Cystideae. They are the Sphaeronites of old authors, and were considered by Professor E. Forbes as intermediate between the crinoids and echinoderms. The Echinosphaeronite here represented (Figure 555) is characteristic of the Caradoc beds in Wales, and of their equivalents in Sweden and Russia.

With it have been found several other genera of the same family, such as Sphaeronites, Hemicosmites, etc. Among the mollusca are Pteropods of the genus Conularia of large size (for genus, see Figure 518). About eleven species of Graptolite are reckoned as belonging to this formation; they are chiefly found in peculiar localities where black mud abounded. The formation, when traced into South Wales and Ireland, a.s.sumes a greatly altered mineral aspect, but still retains its characteristic fossils. The known fauna of the Bala group comprises 565 species, 352 of which are peculiar, and 93, as before stated, are common to the overlying Llandovery rocks. It is worthy of remark that, when it occurs under the form of trappean tuff (volcanic ashes of De la Beche), as in the crest of Snowdon, the peculiar species which distinguish it from the Llandeilo beds are still observable. The formation generally appears to be of shallow-water origin, and in that respect is contrasted with the group next to be described.

Professor Ramsay estimates the thickness of the Bala Beds, including the contemporaneous volcanic rocks, stratified and unstratified, as being from 10,000 to 12,000 feet.

LLANDEILO FLAGS.

(FIGURE 556. Didymograpsus (Graptolites) Murchisonii, Beck. Llandeilo flags, Wales.)

The Lower Silurian strata were originally divided by Sir R. Murchison into the upper group already described, under the name of Caradoc Sandstone, and a lower one, called, from a town in Carmarthens.h.i.+re, the Llandeilo flags. The last mentioned strata consist of dark-coloured micaceous flags, frequently calcareous, with a great thickness of shales, generally black, below them. The same beds are also seen at Builth, in Radnors.h.i.+re, where they are interstratified with volcanic matter.

(FIGURE 557. Diplograpsus pristis, Hisinger. Llandeilo beds, Waterford.)

(FIGURE 558. Rastrites peregrinus, Barrande. Scotland; Bohemia; Saxony.

Llandeilo flags.)

(FIGURE 559. Diplograpsus folium, Hisinger. Dumfriess.h.i.+re; Sweden. Llandeilo flags.)