The Student's Elements of Geology Part 56

We come next in descending order to that division of Primary or Palaeozoic rocks which immediately underlie the Devonian group or Old Red Sandstone. For these strata Sir Roderick Murchison first proposed the name of Silurian when he had studied and cla.s.sified them in that part of Wales and some of the contiguous counties of England which once const.i.tuted the kingdom of the Silures, a tribe of ancient Britons. Table 26.1 will explain the two princ.i.p.al divisions, Upper and Lower, of the Silurian rocks, and the minor subdivisions usually adopted, comprehending all the strata originally embraced in the Silurian system by Sir Roderick Murchison. The formations below the Arenig or Stiper-stones group are treated of in the next chapter, when the "Primordial" or Cambrian group is described.

TABLE 26.1. SILURIAN ROCKS (THICKNESS GIVEN IN FEET).

UPPER SILURIAN ROCKS.

1. LUDLOW FORMATION:

a. Upper Ludlow beds: 780.

b. Lower Ludlow beds: 1,050.

2. WENLOCK FORMATION:

a. Wenlock limestone and shale and b. Woolhope limestone and shale, and Denbighshire grits: above 4,000.

3. LLANDOVERY FORMATION (Beds of pa.s.sage between Upper and Lower Silurian):

a. Upper Llandovery (May-Hill beds): 800.

b. Lower Llandovery: 600-1,000.

LOWER SILURIAN ROCKS.

1. BALA AND CARADOC BEDS, including volcanic rocks: 12,000.

2. LLANDEILO FLAGS, including volcanic rocks: 4,500.

3. ARENIG OR STIPER-STONES GROUP, including volcanic rocks: above 10,000.

UPPER SILURIAN ROCKS.

1. LUDLOW FORMATION.

This member of the Upper Silurian group, as will be seen by Table 26.1, is of great thickness, and subdivided into two parts-- the Upper Ludlow and the Lower Ludlow. Each of these may be distinguished near the town of Ludlow, and at other places in Shropshire and Herefordshire, by peculiar organic remains; but out of more than 500 species found in the Ludlow formation as a whole, not more than five species per hundred are common to the overlying Devonian. The student may refer to the excellent tables given in the last edition of Sir R. Murchison's Siluria for a list of the organic remains of all cla.s.ses distributed through the different subdivisions of the Upper and Lower Silurian.

A. UPPER LUDLOW: DOWNTON SANDSTONE.

At the top of this subdivision there occur beds of fine-grained yellowish sandstone and hard reddish grits which were formerly referred by Sir R.

Murchison to the Old Red Sandstone, under the name of "Tilestones." In mineral character this group forms a transition from the Silurian to the Old Red Sandstone, the strata of both being conformable; but it is now ascertained that the fossils agree in great part specifically, and in general character entirely, with those of the underlying Upper Ludlow rocks. Among these are Orthoceras bullatum, Platyschisma helicites, Bellerophon trilobatus, Chonetes lata, etc., with numerous defenses of fishes.

These beds, therefore, now generally called the "Downton Sandstone," are cla.s.sed as the newest member of the Upper Silurian. They are well seen at Downton Castle, near Ludlow, where they are quarried for building, and at Kington, in Herefordshire. In the latter place, as well as at Ludlow, crustaceans of the genera Pterygotus (for genus see Figure 504) and Eurypterus are met with.

BONE-BED OF THE UPPER LUDLOW.

At the base of the Downton sandstones there occurs a bone-bed which deserves especial notice as affording the most ancient example of fossil fish occurring in any considerable quant.i.ty. It usually consists of one or two thin layers of brown bony fragments near the junction of the Old Red Sandstone and the Ludlow rocks, and was first observed by Sir R. Murchison near the town of Ludlow, where it is three or four inches thick. It has since been traced to a distance of 45 miles from that point into Gloucestershire and other counties, and is commonly not more than an inch thick, but varies to nearly a foot. Near Ludlow two bone- beds are observable, with 14 feet of intervening strata full of Upper Ludlow fossils. (Murchison's Siluria page 140.) At that point immediately above the upper fish-bed numerous small globular bodies have been found, which were determined by Dr. Hooker to be the sporangia of a cryptogamic land-plant, probably lycopodiaceous.

(FIGURE 524. Onchus tenuistriatus, Aga.s.siz. Bone-bed. Upper Silurian. Ludlow.)

(FIGURE 525. s.h.a.green-scales of a placoid fish, Thelodus parvidens, Aga.s.siz.

Bone-bed, Upper Ludlow.)

(FIGURE 526. Plectrodus mirabilis, Aga.s.siz. Bone-bed, Upper Ludlow.)

Most of the fish have been referred by Aga.s.siz to his placoid order, some of them to the genus Onchus, to which the spine (Figure 524) and the minute scales (Figure 525) are supposed to belong. It has been suggested, however, that Onchus may be one of those Acanthodian fish referred by Aga.s.siz to his Ganoid order, which are so characteristic of the base of the Old Red Sandstone in Forfarshire, although the species of the Old Red are all different from these of the Silurian beds now under consideration. The jaw and teeth of another predaceous genus (Figure 526) have also been detected, together with some specimens of Pteraspis Ludensis. As usual in bone-beds, the teeth and bones are, for the most part, fragmentary and rolled.

GREY SANDSTONE AND MUDSTONE, ETC.

(FIGURE 527. Orthis elegantula, Dalm. Var. Orbicularis, Sowerby. Upper Ludlow.)

(FIGURE 528. Rhynchonella navicula, Sowerby. Ludlow Beds.)

The next subdivision of the Upper Ludlow consists of grey calcareous sandstone, or very commonly a micaceous stone, decomposing into soft mud, and contains, besides the sh.e.l.ls mentioned above, Lingula cornea, Orthis...o...b..cularis, a round variety of O. elegantula, Modiolopsis platyphylla, Grammysia cingulata, all characteristic of the Upper Ludlow. The lowest or mud-stone beds contain Rhynchonella navicula (Figure 528), which is common to this bed and the Lower Ludlow. As usual in Palaeozoic strata older than the coal, the brachiopodous mollusca greatly outnumber the lamellibranchiate (see below); but the latter are by no means unrepresented. Among other genera, for example, we observe Avicula and Pterinea, Cardiola, Ctenodonta (sub-genus of Nucula), Orthonota, Modiolopsis, and Palaearca.

Some of the Upper Ludlow sandstones are ripple-marked, thus affording evidence of gradual deposition; and the same may be said of the accompanying fine argillaceous shales, which are of great thickness, and have been provincially named "mud-stones." In some of these shales stems of crinoidea are found in an erect position, having evidently become fossil on the spots where they grew at the bottom of the sea. The facility with which these rocks, when exposed to the weather, are resolved into mud, proves that, notwithstanding their antiquity, they are nearly in the state in which they were first thrown down.

b. LOWER LUDLOW BEDS.

(FIGURE 529. Pentamerus Knightii, Sowerby. Aymestry. One-half natural size.

a. View of both valves united.

b. Longitudinal section through both valves, showing the central plates or septa.)

The chief ma.s.s of this formation consists of a dark grey argillaceous shale with calcareous concretions, having a maximum thickness of 1000 feet. In some places, and especially at Aymestry, in Herefordshire, a subcrystalline and argillaceous limestone, sometimes 50 feet thick, overlies the shale. Sir R. Murchison therefore cla.s.ses this Aymestry limestone as holding an intermediate position between the Upper and Lower Ludlow, but Mr. Lightbody remarks that at Mocktrie, near Leintwardine, the Lower Ludlow shales, with their characteristic fossils, occur both above and below a similar limestone. This limestone around Aymestry and Sedgeley is distinguished by the abundance of Pentamerus Knightii, Sowerby (Figure 529), also found in the Lower Ludlow and Wenlock shale. This genus of brachiopoda was first found in Silurian strata, and is exclusively a palaeozoic form. The name was derived from pente, five, and meros, a part, because both valves are divided by a central septum, making four chambers, and in one valve the septum itself contains a small chamber, making five. The size of these septa is enormous compared with those of any other brachiopod sh.e.l.l; and they must nearly have divided the animal into two equal halves; but they are, nevertheless, of the same nature as the septa or plates which are found in the interior of Spirifera, Terebratula, and many other sh.e.l.ls of this order. Messrs.

Murchison and De Verneuil discovered this species dispersed in myriads through a white limestone of Upper Silurian age, on the banks of the Is, on the eastern flank of the Urals in Russia, and a similar species is frequent in Sweden.

(FIGURE 530. Lingula Lewisii, J. Sowb. Abberley Hills.)

(FIGURE 531. Rhynchonella (Terebratula) Wilsoni, Sowerby. Aymestry.)

(FIGURE 532. Atrypa reticularis, Linn. (Terebratula affinis, Min. Con.) Aymestry.

a. Upper valve.

b. Lower valve.

c. Anterior margin of the valves.)

Three other abundant sh.e.l.ls in the Aymestry limestone are, 1st, Lingula Lewisii (Figure 530); second, Rhynchonella Wilsoni, Sowerby. (Figure 531), which is also common to the Lower Ludlow and Wenlock limestone; third, Atrypa reticularis, Linn. (Figure 532), which has a very wide range, being found in every part of the Upper Silurian system, and even ranging up into the Middle Devonian series.

The Aymestry Limestone contains many sh.e.l.ls, especially brachiopoda, corals, trilobites, and other fossils, amounting on the whole to 74 species, all except three or four being common to the beds either above or below.

(FIGURE 533. Phragmoceras ventricosum, J. Sowerby. (Orthoceras ventricosum, Stein.) Aymestry; one-quarter natural size.)

(FIGURE 534. Lituites (Trochoceras) giganteus, J. Sowerby. Near Ludlow; also in the Aymestry and Wenlock Limestones; 1/4 natural size.)

(FIGURE 535. Fragment of Orthoceras Ludense, J. Sowerby. Leintwardine, Shropshire.)

The Lower Ludlow Shale contains, among other fossils, many large cephalopoda not known in newer rocks, as the Phragmoceras of Broderip, and the Lituites of Breynius (see Figures 533, 534). The latter is partly straight and partly convoluted in a very flat spire. The Orthoceras Ludense (Figure 535), as well as the cephalopod last mentioned, occurs in this member of the species.

A species of Graptolite, G. priodon, Bronn (Figure 545), occurs plentifully in the Lower Ludlow. This fossil, referred, though somewhat doubtfully, to a form of hydrozoid or sertularian polyp, has not yet been met with in strata above the Silurian.

Star-fish, as Sir R. Murchison points out, are by no means rare in the Lower Ludlow rock. These fossils, of which six extinct genera are now known in the Ludlow series, represented by 18 species, remind us of various living forms now found in our British seas, both of the families Asteriadae and Ophiuridae.

OLDEST KNOWN FOSSIL FISH.