The Student's Elements of Geology Part 59

A still lower part of the Llandeilo rocks consists of a black carbonaceous slate of great thickness, frequently containing sulphate of alumina, and sometimes, as in Dumfriesshire, beds of anthracite. It has been conjectured that this carbonaceous matter may be due in great measure to large quant.i.ties of imbedded animal remains, for the number of Graptolites included in these slates was certainly very great. In Great Britain eleven genera and about 40 species of Graptolites occur in the Llandeilo flags and underlying Arenig beds. The double Graptolites, or those with two rows of cells, such as Diplograpsus (Figure 557), are conspicuous.

The brachiopoda of the Llandeilo flags, which number 47 species, are in the main the same as those of the Caradoc Sandstone, but the other mollusca are in great part of different species.

(FIGURE 560. Orthoceras duplex, Wahlenberg. Russia and Sweden. (From Murchison's Siluria.))

(FIGURE 561. Asaphus tyrannus, Murchison. Llandeilo; Bishop's Castle; etc.)

(FIGURE 562. Ogygia Buchii, Burm. Syn. Asaphus Buchii, Brongn. Builth, Radnorshire; Llandeilo, Carmarthenshire.)

In Europe generally, as, for example, in Sweden and Russia, no sh.e.l.ls are so characteristic of this formation as Orthocerat.i.tes, usually of great size, and with a wide siphuncle placed on one side instead of being central (see Figure 560). Among other Cephalopods in the Llandeilo flags is Cyrtoceras; in the same beds also are found Bellerophon (see Figure 488) and some Pteropod sh.e.l.ls (Conularia, Theca, etc.), also in spots where sand abounded, lamellibranchiate bivalves of large size. The Crustaceans were plentifully represented by the Trilobites, which appear to have swarmed in the Silurian seas just as crabs and shrimps do in our own; no less than 263 species have been found in the British Silurian fauna. The genera Asaphus (Figure 561), Ogygia (Figure 562), and Trinucleus (Figures 552 and 553) form a marked feature of the rich and varied Trilobitic fauna of this age.

Beneath the black slates above described of the Llandeilo formation, Graptolites are still found in great variety and abundance, and the characteristic genera of sh.e.l.ls and trilobites of the Lower Silurian rocks are still traceable downward, in Shropshire, c.u.mberland, and North and South Wales, through a vast depth of shaly beds, in some districts interstratified with trappean formations of contemporaneous origin; these consist of tuffs and lavas, the tuffs being formed of such materials as are ejected from craters and deposited immediately on the bed of the ocean, or washed into it from the land. According to Professor Ramsay, their thickness is about 3300 feet in North Wales, including those of the Lower Llandeilo. The lavas are feldspathic, and of porphyritic structure, and, according to the same authority, of an aggregate thickness of 2500 feet.

ARENIG OR STIPER-STONES GROUP (LOWER LLANDEILO OF MURCHISON).

(FIGURE 563. Arenicolites linearis, Hall. Arenig beds, Stiper-Stones.

a. Parting between the beds, or planes of bedding.)

(FIGURE 564. Didymograpsus geminus, Hisinger, sp. Sweden.)

Next in the descending order are the shales and sandstones in which the quartzose rocks called Stiper-Stones in Shropshire occur. Originally these Stiper-Stones were only known as arenaceous quartzose strata in which no organic remains were conspicuous, except the tubular burrows of annelids (see Figure 563, Arenicolites linearis), which are remarkably common in the Lowest Silurian in Shropshire, and in the State of New York, in America. They have already been alluded to as occurring by thousands in the Silurian strata unconformably overlying the Cambrian, in the mountain of Queenaig, in Sutherlandshire (Figure 82). I have seen similar burrows now made on the retiring of the tides in the sands of the Bristol Channel, near Minehead, by lob-worms which are dug out by fishermen and used as bait. When the term Silurian was given by Sir R.

Murchison, in 1835, to the whole series, he considered the Stiper-Stones as the base of the Silurian system, but no fossil fauna had then been obtained, such as could alone enable the geologist to draw a line between this member of the series and the Llandeilo flags above, or a vast thickness of rock below, which was seen to form the Longmynd hills, and was called "unfossiliferous graywacke."

Professor Sedgwick had described, in 1843, strata now ascertained to be of the same age as largely developed in the Arenig mountain, in Merionethshire; and the Skiddaw slates in the Lake-District of c.u.mberland, studied by the same author, were of corresponding date, though the number of fossils was, in both cases, too few for the determination of their true chronological relations. The subsequent researches of Messrs. Sedgwick and Harkness, in c.u.mberland, and of Sir R.I.

Murchison and the Government surveyors in Shropshire, have increased the species to more than sixty. These were examined by Mr. Salter, and shown in the third edition of "Siluria" (page 52, 1859) to be quite distinct from the fossils of the overlying Llandeilo flags. Among these the Obolella plumbea, Aeglina binodosa, Ogygia Selwynii, and Didymograpsus geminus (Figure 564), and D.

Hirundo, are characteristic.

But, although the species are distinct, the genera are the same as those which characterise the Silurian rocks above, and none of the characteristic primordial or Cambrian forms, presently to be mentioned, are intermixed. The same may be said of a set of beds underlying the Arenig rocks at Ramsay Island and other places in the neighbourhood of St. David's. These beds, which have only lately become known to us through the labours of Dr. Hicks (Transactions of the British a.s.sociation 1866. Proceedings of the Liverpool Geological Society 1869.), present already twenty new species, the greater part of them allied generically to the Arenig rocks. This Arenig group may therefore be conveniently regarded as the base of the great Silurian system, a system which, by the thickness of its strata and the changes in animal life of which it contains the record, is more than equal in value to the Devonian, or Carboniferous, or other princ.i.p.al divisions, whether of primary or secondary date.

It would be unsafe to rely on the mere thickness of the strata, considered apart from the great fluctuations in organic life which took place between the era of the Llandeilo and that of the Ludlow formation, especially as the enormous pile of Silurian rocks observed in Great Britain (in Wales more particularly) is derived in great part from igneous action, and is not confined to the ordinary deposition of sediment from rivers or the waste of cliffs.

In volcanic archipelagoes, such as the Canaries, we see the most active of all known causes, aqueous and igneous, simultaneously at work to produce great results in a comparatively moderate lapse of time. The outpouring of repeated streams of lava-- the showering down upon land and sea of volcanic ashes-- the sweeping seaward of loose sand and cinders, or of rocks ground down to pebbles and sand, by rivers and torrents descending steeply inclined channels-- the undermining and eating away of long lines of sea-cliff exposed to the swell of a deep and open ocean-- these operations combine to produce a considerable volume of superimposed matter, without there being time for any extensive change of species. Nevertheless, there would seem to be a limit to the thickness of stony ma.s.ses formed even under such favourable circ.u.mstances, for the a.n.a.logy of tertiary volcanic regions lends no countenance to the notion that sedimentary and igneous rocks 25,000, much less 45,000 feet thick, like those of Wales, could originate while one and the same fauna should continue to people the earth. If, then, we allow that about 25,000 feet of matter may be ascribed to one system, such as the Silurian, as above described, we may be prepared to discover in the next series of subjacent rocks a distinct a.s.semblage of species, or even in great part of genera, of organic remains. Such appears to be the fact, and I shall therefore conclude with the Arenig beds my enumeration of the Silurian formations in Great Britain, and proceed to say something of their foreign equivalents, before treating of rocks older than the Silurian.

SILURIAN STRATA OF THE CONTINENT OF EUROPE.

When we turn to the continent of Europe, we discover the same ancient series occupying a wide area, but in no region as yet has it been observed to attain great thickness. Thus, in Norway and Sweden, the total thickness of strata of Silurian age is considerably less than 1000 feet, although the representatives both of the Upper and Lower Silurian of England are not wanting there. In Russia the Silurian strata, so far as they are yet known, seem to be even of smaller vertical dimensions than in Scandinavia, and they appear to consist chiefly of the Llandovery group, or of a limestone containing Pentamerus oblongus, below which are strata with fossils corresponding to those of the Llandeilo beds of England. The lowest rock with organic remains yet discovered is "the Ungulite or Obolus grit" of St. Petersburg, probably coeval with the Llandeilo flags of Wales.

(Figures 565 and 566. Sh.e.l.ls of the lowest known Fossiliferous Beds in Russia.

(FIGURE 565. Siphonotreta unguiculata, Eichwald. From the Lowest Silurian Sandstone, "Obolus grits," of St. Petersburg.

a. Outside of perforated valve.

b. Interior of same, showing the termination of the foramen within. (Davidson.))

(FIGURE 566. Obolus Apollinis, Eichwald. From the same locality.

a. Interior of the larger or ventral valve.

b. Exterior of the upper (dorsal) valve. (Davidson, "Palaeontographic Monograph.")))

The shales and grits near St. Petersburg, above alluded to, contain green grains in their sandy layers, and are in a singularly unaltered state, taking into account their high antiquity. The prevailing Brachiopods consist of the Obolus or Ungulite of Pander, and a Siphonotreta (Figures 565, 566). Notwithstanding the antiquity of this Russian formation, it should be stated that both of these genera of brachiopods have been also found in the Upper Silurian of England, i.e. In the Wenlock limestone.

Among the green grains of the sandy strata above-mentioned, Professor Ehrenberg announced in 1854 his discovery of remains of foraminifera. These are casts of the cells; and among five or six forms three are considered by him as referable to existing genera (e.g., Textularia, Rotalia, and Guttulina).

SILURIAN STRATA OF THE UNITED STATES.

Table 26.3. SUBDIVISIONS OF THE SILURIAN STRATA OF NEW YORK. (Strata below the Oriskany sandstone or base of the Devonian.)

COLUMN 1: NEW YORK NAMES.

COLUMN 2: BRITISH EQUIVALENTS.

1. Upper Pentamerus Limestone: Upper Silurian (or Ludlow and Wenlock formations).

2. Encrinal Limestone: Upper Silurian (or Ludlow and Wenlock formations).

3. Delthyris Shaly Limestone: Upper Silurian (or Ludlow and Wenlock formations).

4. Pentamerus and Tentaculite Limestones: Upper Silurian (or Ludlow and Wenlock formations).

5. Water Lime Group: Upper Silurian (or Ludlow and Wenlock formations).

6. Onondaga Salt Group: Upper Silurian (or Ludlow and Wenlock formations).

7. Niagara Group: Upper Silurian (or Ludlow and Wenlock formations).

8. Clinton Group: Beds of Pa.s.sage, Llandovery Group.

9. Medina Sandstone: Beds of Pa.s.sage, Llandovery Group.

10. Oneida Conglomerate: Beds of Pa.s.sage, Llandovery Group.

11. Gray Sandstone: Beds of Pa.s.sage, Llandovery Group.

12. Hudson River Group: Lower Silurian (or Caradoc and Bala, Llandeilo and Arenig Formations).

13. Trenton Limestone: Lower Silurian (or Caradoc and Bala, Llandeilo and Arenig Formations).

14. Black-River Limestone: Lower Silurian (or Caradoc and Bala, Llandeilo and Arenig Formations).

15. Bird's-eye Limestone: Lower Silurian (or Caradoc and Bala, Llandeilo and Arenig Formations).

16. Chazy Limestone: Lower Silurian (or Caradoc and Bala, Llandeilo and Arenig Formations).

17. Calciferous Sandstone: Lower Silurian (or Caradoc and Bala, Llandeilo and Arenig Formations).

The Silurian formations can be advantageously studied in the States of New York, Ohio, and other regions north and south of the great Canadian lakes. Here they are often found, as in Russia, nearly in horizontal position, and are more rich in well-preserved fossils than in almost any spot in Europe. In the State of New York, where the succession of the beds and their fossils have been most carefully worked out by the Government surveyors, the subdivisions given in the first column of Table 26.3 have been adopted.

In the second column of the same table I have added the supposed British equivalents. All Palaeontologists, European and American, such as MM. De Verneuil, D. Sharpe, Professor Hall, E. Billings, and others, who have entered upon this comparison, admit that there is a marked general correspondence in the succession of fossil forms, and even species, as we trace the organic remains downward from the highest to the lowest beds; but it is impossible to parallel each minor subdivision.

That the Niagara Limestone, over which the river of that name is precipitated at the great cataract, together with its underlying shales, corresponds to the Wenlock limestone and shale of England there can be no doubt. Among the species common to this formation in America and Europe are Calymene Blumenbachii, Homalonotus delphinocephalus (Figure 544), with several other trilobites; Rhynchonella Wilsoni, Figure 531, and Retzia cuneata; Orthis elegantula, Pentamerus galeatus, with many more brachiopods; Orthoceras annulatum, among the cephalopodous sh.e.l.ls; and Favosites gothlandica, with other large corals.