(9) "Guide to the Geology of New York," Lincklaen; and "Contributions to the Palaeontology of New York," James Hall.--'Fourteenth Report on the State Cabinet.' 1861.

(10) 'Palaeozoic Fossils of Canada.' Billings. 1865.

(11) 'Manual of Geology.' Dana. Pp. 166-182. 2d ed. 1875.

(12) "Geology of North Wales," Ramsay; with Appendix on the Fossils, Salter.--'Memoirs of the Geological Survey of Great Britain,' vol. iii. 1866.

(13) "On the Ancient Rocks of the St David's Promontory, South Wales, and their Fossil Contents." Harkness and Hicks.--' Quart.

Journ. Geol. Soc.,' xxvii. 384-402. 1871.

(14) "On the Tremadoc Rocks in the Neighbourhood of St David's, South Wales, and their Fossil Contents." Hicks.--'Quart.

Journ. Geol. Soc.,' xxix. 39-52. 1873.

In the above list, allusion has necessarily been omitted to numerous works and memoirs on the Cambrian deposits of Sweden and Norway, Central Europe, Russia, Spain, and various parts of North America, as well as to a number of important papers on the British Cambrian strata by various well-known observers. Amongst these latter may be mentioned memoirs by Prof. Phillips, and Messrs Salter, Hicks, Belt, Plant, Homfray, Ash, Holl, &c.

CHAPTER IX.

THE LOWER SILURIAN PERIOD.

The great system of deposits to which Sir Roderick Murchison applied the name of "Silurian Rocks" reposes directly upon the highest Cambrian beds, apparently without any marked unconformity, though with a considerable change in the nature of the fossils. The name "Silurian" was originally proposed by the eminent geologist just alluded to for a great series of strata lying below the Old Red Sandstone, and occupying districts in Wales and its borders which were at one time inhabited by the "Silures," a tribe of ancient Britons. Deposits of a corresponding age are now known to be largely developed in other parts of England, in Scotland, and in Ireland, in North America, in Australia, in India, in Bohemia, Saxony, Bavaria, Russia, Sweden and Norway, Spain, and in various other regions of less note. In some regions, as in the neighbourhood of St Petersburg, the Silurian strata are found not only to have preserved their original horizontality, but also to have retained almost unaltered their primitive soft and incoherent nature. In other regions, as in Scandinavia and many parts of North America, similar strata, now consolidated into shales, sandstones, and limestones, may be found resting with a very slight inclination on still older sediments. In a great many regions, however, the Silurian deposits are found to have undergone more or less folding, crumpling, and dislocation, accompanied by induration and "cleavage" of the finer and softer sediments; whilst in some regions, as in the Highlands of Scotland, actual "metamorphism" has taken place. In consequence of the above, Silurian districts usually present the bold, rugged, and picturesque outlines which are characteristic of the older "Primitive" rocks of the earth's crust in general. In many instances, we find Silurian strata rising into mountain-chains of great grandeur and sublimity, exhibiting the utmost diversity of which rock-scenery is capable, and delighting the artist with endless changes of valley, lake, and cliff. Such districts are little suitable for agriculture, though this is often compensated for by the valuable mineral products contained in the rocks. On the other hand, when the rocks are tolerably soft and uniform in their nature, or when few disturbances of the crust of the earth have taken place, we may find Silurian areas to be covered with an abundant pasturage or to be heavily timbered.

Under the head of "Silurian Rocks," Sir Roderick Murchison included all the strata between the summit of the "Longmynd." beds and the Old Red Sandstone, and he divided these into the two great groups of the _Lower_ Silurian and _Upper_ Silurian. It is, however, now generally admitted that a considerable portion of the bas.e.m.e.nt beds of Murchison's Silurian series must be transferred---if only upon palaeontological grounds--to the Upper Cambrian, as has here been done; and much controversy has been carried on as to the proper nomenclature of the Upper Silurian and of the remaining portion of Murchison's Lower Silurian. Thus, some would confine the name "Silurian" exclusively to the Upper Silurian, and would apply the name of "Cambro-Silurian" to the Lower Silurian, or would include all beds of the latter age in the "Cambrian" series of Sedgwick.

It is not necessary to enter into the merits of these conflicting views. For our present purpose, it is sufficient to recognise that there exist two great groups of rocks between the highest Cambrian beds, as here defined, and the base of the Devonian or Old Red Sandstone. These two great groups are so closely allied to one another, both physically and palaeontologically, that many authorities have established a third or intermediate group (the "Middle Silurian"), by which a pa.s.sage is made from one into the other. This method of procedure involves disadvantages which appear to outweigh its advantages; and the two groups in question are not only generally capable of very distinct stratigraphical separation, but at the same time exhibit, together with the alliances above spoken of, so many and such important palaeontological differences, that it is best to consider them separately. We shall therefore follow this course in the present instance; and pending the final solution of the controversy as to Cambrian and Silurian nomenclature, we shall distinguish these two groups of strata as the "Lower Silurian" and the "Upper Silurian."

The _Lower Silurian Rocks_ are known already to be developed in various regions; and though their _general_ succession in these areas is approximately the same, each area exhibits peculiarities of its own, whilst the subdivisions of each are known by special names. All, therefore, that can be attempted here, is to select two typical areas--such as Wales and North America and to briefly consider the grouping and divisions of the Lower Silurian in each.

In Wales, the line between the Cambrian and Lower Silurian is somewhat ill-defined, and is certainly not marked by any strong unconformity. There are, however; grounds for accepting the line proposed, for palaeontological reasons, by Dr Hicks, in accordance with which the Tremadoc Slates ("Lower Tremadoc" of Salter) become the highest of the Cambrian deposits of Britain. If we take this view, the Lower Silurian rocks of Wales and adjoining districts are found to have the following _general_ succession from below upwards (fig. 34):--

1. The _Arenig Group_.--This group derives its name from the Arenig mountains, where it is extensively developed. It consists of about 4000 feet of slates, shales, and flags, and is divisible into a lower, middle, and upper division, of which the former is often regarded as Cambrian under the name of "Upper Tremadoc Slates."

2. The _Llandeilo Group_.--The thickness of this group varies from about 4000 to as much as 10,000 feet; but in this latter case a great amount of the thickness is made up of volcanic ashes and interbedded traps. The sedimentary beds of this group are princ.i.p.ally slates and flags, the latter occasionally with calcareous bands; and the whole series can be divided into a lower, middle, and upper Llandeilo division, of which the last is the most important. The name of "Llandeilo" is derived from the town of the same name in Wales, where strata of this age were described by Murchison.

3. The _Caradoc_ or _Bala Group_.--The alternative names of this group are also of local origin, and are derived, the one from Caer Caradoc in Shropshire, the other from Bala in Wales, strata of this age occurring in both localities. The series is divided into a lower and upper group, the latter chiefly composed of shales and flags, and the former of sandstones and shales, together with the important and interesting calcareous band known as the "Bala Limestone." The thickness of the entire series varies from 4000 to as much as 12,000 feet, according as it contains more or less of interstratified igneous rocks.

4. The _Llandovery Group_ (Lower Llandovery of Murchison).--This series, as developed near the town of Llandovery, in Caermarthenshire, consists of less than 1000 feet of conglomerates, sandstones, and shales. It is probable, however, that the little calcareous band known as the "Hirnant Limestone," together with certain pale-coloured slates which lie above the Bala Limestone, though usually referred to the Caradoc series, should in reality be regarded as belonging to the Llandovery group.

The general succession of the Lower Silurian strata of Wales and its borders, attaining a maximum thickness (along with contemporaneous igneous matter) of nearly 30,000 feet, is diagramatically represented in the annexed sketch-section (fig.

34):--

[Ill.u.s.tration: Fig 34. GENERALIZED SECTION OF THE LOWER SILURIAN ROCKS OF WALES.]

In North America, both in the United States and in Canada, the Silurian rocks are very largely developed, and may be regarded as const.i.tuting an exceedingly full and typical series of the deposits of this period. The chief groups of the Silurian rocks of North America are as follows, beginning, as before, with the lowest strata, and proceeding upwards (fig. 35):--

1. _Quebec Group_.--This group is typically developed in the vicinity of Quebec, where it consists of about 5000 feet of strata, chiefly variously-coloured shales, together with some sandstones and a few calcareous bands. It contains a number of peculiar Graptolites, by which it can be identified without question with the Arenig group of Wales and the corresponding Skiddaw Slates of the North of England. It is also to be noted that numerous Trilobites of a distinct Cambrian _facies_ have been obtained in the limestones of the Quebec group, near Quebec. These fossils, however, have been exclusively obtained from the limestones of the group; and as these limestones are princ.i.p.ally calcareous breccias or conglomerates, there is room for believing that these primordial fossils are really derived, in part at any rate, from fragments of an upper Cambrian limestone. In the State of New York, the Graptolitic shales of Quebec are wanting; and the base of the Silurian is const.i.tuted by the so-called "Calciferous Sand-rock" and "Chazy Limestone."[11] The first of these is essentially and typically calcareous, and the second is a genuine limestone.

[Footnote 11: The precise relations of the Quebec shales with Graptolites (Levis Formation) to the Calciferous and Chazy beds are still obscure, though there seems little doubt but that the Quebec Shales are superior to the Calciferous Sand-rock.]

2. The _Trenton Group_.--This is an essentially calcareous group, the various limestones of which it is composed being known as the "Bird's-eye," "Black River," and "Trenton" Limestones, of which the last is the thickest and most important. The thickness of this group is variable, and the bands of limestone in it are often separated by beds of shale.

3. The _Cincinnati Group_ (Hudson River Formation[12]).--This group consists essentially of a lower series of shales, often black in colour and highly charged with bituminous matter (the "Utica Slates "), and of an upper series of shales, sandstones, and limestones (the "Cincinnati" rocks proper). The exact parallelism of the Trenton and Cincinnati groups with the subdivisions of the Welsh Silurian series can hardly be stated positively. Probably no precise equivalency exists; but there can be no doubt but that the Trenton and Cincinnati groups correspond, as a whole, with the Llandeilo and Caradoc groups of Britain. The subjoined diagrammatic section (fig. 35) gives a general idea of the succession of the Lower Silurian rocks of North America:--

[Ill.u.s.tration: Fig 35. GENERALIZED SECTION OF THE LOWER SILURIAN ROCKS OF NORTH AMERICA.]

[Ill.u.s.tration: Fig. 36.--_Licrophycus Ottawaensis_ a "Fucoid,"

from the Trenton Limestone (Lower Silurian) of Canada. (After Billings.)]

[Footnote 12: There is some difficulty about the precise nomenclature of this group. It was originally called the "Hudson River Formation;"

but this name is inappropriate, as rocks of this age hardly touch anywhere the actual Hudson River itself, the rocks so called formerly being now known to be of more ancient date. There is also some want of propriety in the name of "Cincinnati Group,"

since the rocks which are known under this name in the vicinity of Cincinnati itself are the representatives of the Trenton Limestone, Utica Slates, and the old Hudson River group, inseparably united in what used to be called the "Blue Limestone Series."].

Of the _life_ of the Lower Silurian period we have record in a vast number of fossils, showing that the seas of this period were abundantly furnished with living denizens. We have, however, in the meanwhile, no knowledge of the land-surfaces of the period.

We have therefore no means of speculating as to the nature of the terrestrial animals of this ancient age, nor is anything known with certainty of any land-plants which may have existed.

The only relics of vegetation upon which a positive opinion can be expressed belong to the obscure group of the "Fucoids," and are supposed to be the remains of sea-weeds. Some of the fossils usually placed under this head are probably not of a vegetable nature at all, but others (fig. 36) appear to be unquestionable plants. The true affinities of these, however, are extremely dubious. All that can be said is, that remains which appear to be certainly vegetable, and which are most probably due to marine plants, have been recognised nearly at the base of the Lower Silurian (Arenig), and that they are found throughout the series whenever suitable conditions recur.

The Protozoans appear to have flourished extensively in the Lower Silurian seas, though to a large extent under forms which are still little understood. We have here for the first time the appearance of Foraminifera of the ordinary type--one of the most interesting observations in this collection being that made by Ehrenberg, who showed that the Lower Silurian sandstones of the neighbourhood of St Petersburg contained casts in glauconite of Foraminiferous sh.e.l.ls, some of which are referable to the existing genera _Rotalia_ and _Texularia_. True _Sponges_, belonging to that section of the group in which the skeleton is calcareous, are also not unknown, one of the most characteristic genera being _Astylospongia_ (fig. 37). In this genus are included more or less globular, often lobed sponges, which are believed not to have been attached to foreign bodies. In the form here figured there is a funnel-shaped cavity at the summit; and the entire ma.s.s of the sponge is perforated, as in living examples, by a system of ca.n.a.ls which convey the sea-water to all parts of the organism. The ca.n.a.ls by which the sea-water gains entrance open on the exterior of the sphere, and those by which it again escapes from the sponge open into the cup-shaped depression at the summit.

[Ill.u.s.tration: Fig. 37.--_Astylospongia proemorsa_, cut vertically so as to exhibit the ca.n.a.l-system in the interior. Lower Silurian, Tennessee. (After Ferdinand Roemer.)]

The most abundant, and at the same time the least understood, of Lower Silurian Protozoans belong, however, to the genera _Stromatopora_ and _Receptaculites_, the structure of which can merely be alluded to here. The specimens of _Stromatopora_ (fig.

38) occur as hemispherical, pear-shaped, globular, or irregular ma.s.ses, often of very considerable size, and sometimes demonstrably attached to foreign bodies. In their structure these ma.s.ses consist of numerous thin calcareous laminae, usually arranged concentrically, and separated by narrow inters.p.a.ces. These inters.p.a.ces are generally crossed by numerous vertical calcareous pillars, giving the vertical section of the fossil a lattice-like appearance. There are also usually minute pores in the concentric laminae, by which the successive inters.p.a.ces are placed in communication; and sometimes the surface presents large rounded openings, which appear to correspond with the water-ca.n.a.ls of the Sponges. Upon the whole, though presenting some curious affinities to the calcareous Sponges, _Stromatopora_ is perhaps more properly regarded as a gigantic _Foraminifer_. If this view be correct, it is of special interest as being probably the nearest ally of _Eozoon_, the general appearance of the two being strikingly similar, though their minute structure is not at all the same. Lastly, in the fossils known as _Receptaculites_ and _Ischadites_ we are also presented with certain singular Lower Silurian Protozoans, which may with great probability be regarded as gigantic _Foraminifera_. Their structure is very complex; but fragments are easily recognised by the fact that the exterior is covered with numerous rhomboidal calcareous plates, closely fitting together, and arranged in peculiar intersecting curves, presenting very much the appearance of the engine-turned case of a watch.

[Ill.u.s.tration: Fig. 38.--A small and perfect specimen of _Stromatopora rugosa_, of the natural size, from the Trenton Limestone of Canada. (After Billings.)]

Pa.s.sing next to the sub-kingdom of _Coelenterate_ animals (Zoophytes, Corals, &c.), we find that this great group, almost or wholly absent in the Cambrian, is represented in Lower Silurian deposits by a great number of forms belonging on the one hand to the true Corals, and en the other hand to the singular family of the _Graptolites_. If we except certain plant-like fossils which probably belong rather to the Sertularians or the Polyzoans (e.g., _Dictyonema, Dendrograptus_, &c.), the family of the _Graptolites_ may be regarded as exclusively Silurian in its distribution. Not only is this the case, but it attained its maximum development almost upon its first appearance, in the Arenig Rocks; and whilst represented by a great variety of types in the Lower Silurian; it only exists in the Upper Silurian in a much diminished form.

The _Graptolites_ (Gr. _grapho_, I write; _lithos_, stone) were so named by Linnaeus, from the resemblance of some of them to written or pencilled marks upon the stone, though the great naturalist himself did not believe them to be true fossils at all. They occur as linear or leaf-like bodies, sometimes simple, sometimes compound and branched; and no doubt whatever can be entertained as to their being the skeletons of composite organisms, or colonies of semi-independent animals united together by a common fleshy trunk, similar to what is observed in the colonies of the existing Sea-firs (Sertularians). This fleshy trunk or common stem of the colony was protected by a delicate h.o.r.n.y sheath, and it gave origin to the little flower-like "polypites," which const.i.tuted the active element of the whole a.s.semblage. These semi-independent beings were, in turn, protected each by a little h.o.r.n.y cup or cell, directly connected with the common sheath below, and terminating above in an opening through which the polypite could protrude its tentacled head or could again withdraw itself for safety. The entire skeleton, again, was usually, if not universally, supported by a delicate h.o.r.n.y rod or "axis,"

which appears to have been hollow, and which often protrudes to a greater or less extent beyond one or both of the extremities of the actual colony.

The above gives the elementary const.i.tution of any _Graptolite_, but there are considerable differences as to the manner in which these elements are arranged and combined. In some forms the common stem of the colony gives origin to but a single row of cells on one side. If the common stem is a simple, straight, or slightly-curved linear body, then we have the simplest form of Graptolite known (the genus _Monograptus_); and it is worthy of note that these simple types do not come into existence till comparatively late (Llandeilo), and last nearly to the very close of the Upper Silurian. In other cases, whilst there is still but a single row of cells, the colony may consist of two of these simple stems springing from a common point, as in the so-called "twin Graptolites" (_Didymograptus_, fig. 40). This type is entirely confined to the earlier portion of the Lower Silurian period (Arenig and Llandeilo). In other cases, again, there may be four of such stems springing from a central point (_Tetragraptus_).

Lastly, there are numerous complex forms (such as _Dichograptus, Loganograptus_, &c.) in which there are eight or more of these simple branches, all arising from a common centre (fig. 39), which is sometimes furnished with a singular h.o.r.n.y disc. These complicated branching forms, as well as the _Tetragrapti_, are characteristic of the horizon of the Arenig group. Similar forms, often specifically identical, are found at this horizon in Wales, in the great series of the Skiddaw Slates of the north of England, in the Quebec group in Canada, in equivalent beds in Sweden, and in certain gold-bearing slates of the same age in Victoria in Australia.

[Ill.u.s.tration: Fig. 39.--_Dichograptus octobrachiatus_, a branched, "unicellular" Graptolite from the Skiddaw and Quebec Groups (Arenig).

(After Hall.)]

In another great group of Graptolites (including the genera _Diplograptus, Dicranograptus, Climacograptus_, &c.) the common stem of the colony gives origin, over part or the whole or its length, to _two_ rows of cells, one on each side (fig. 41). These "double-celled" Graptolites are highly characteristic of the Lower Silurian deposits; and, with an exception more apparent than real in Bohemia, they are exclusively confined to strata of Lower Silurian age, and are not known to occur in the Upper Silurian.

Lastly, there is a group of Graptolites (_Phyllograptus_, fig.

42) in which the colony is leaf-like in form, and is composed of _four_ rows of cells springing in a cross-like manner from the common stem. These forms are highly characteristic of the Arenig group.

[Ill.u.s.tration: Fig. 40.--Central portion of the colony of _Didymegraptus divaricatus_, Upper Llandeilo, Dumfresshire.

(Original.)]

[Ill.u.s.tration: Fig. 41.--Examples of _Diplograptus pristis_, showing variations in the appendages at the base. Upper Llandeilo, Dumfriesshire. (Original.)]

[Ill.u.s.tration: Fig. 42.--Group of individuals of _Phyllograptus typus_, from the Quebec group of Canada. (After Hall.) One of the four rows of cells is hidden on the under surface.]

The Graptolites are usually found in dark-coloured, often black shales, which sometimes contain so much carbon as to become "anthracitic." They may be simply carbonaceous; but they are more commonly converted into iron-pyrites, when they glitter with the brilliant l.u.s.tre of silver as they lie scattered on the surface of the rock, fully deserving in their metallic tracery the name of "written stones." They const.i.tute one of the most important groups of Silurian fossils, and are of the greatest value in determining the precise stratigraphical position of the beds in which they occur. They present, however, special difficulties in their study; and it is still a moot point as to their precise position in the zoological scale. The balance of evidence is in favour of regarding them as an ancient and peculiar group of the Sea-firs (Hydroid Zoophytes), but some regard them as belonging rather to the Sea-mosses (_Polyzoa_).

Under any circ.u.mstances, they cannot be directly compared either with the ordinary Sea-firs or the ordinary Sea-mosses; for these two groups consist of fixed organisms, whereas the Graptolites were certainly free-floating creatures, living at large in the open sea. The only Hydroid Zoophytes or Polyzoans which have a similar free mode of existence, have either no skeleton at all, or have hard structures quite unlike the h.o.r.n.y sheaths of the Graptolites.

The second great group of Coelenterate animals (_Actinozoa_) is represented in the Lower Silurian rocks by numerous Corals.

These, for obvious reasons, are much more abundant in regions where the Lower Silurian series is largely calcareous (as in North America) than in districts like Wales, where limestones are very feebly developed. The Lower Silurian Corals, though the first of their cla.s.s, and presenting certain peculiarities, may be regarded as essentially similar in nature to existing Corals. These, as is well known, are the calcareous skeletons of animals--the so-called "Coral-Zoophytes"--closely allied to the common Sea-anemones in structure and habit. A _simple_ coral (fig.

43) consists of a calcareous cup embedded in the soft tissues of the flower-like polype, and having at its summit a more or less deep depression (the "calice") in which the digestive organs are contained. The s.p.a.ce within the coral is divided into compartments by numerous vertical calcareous plates (the "septa"), which spring from the inside of the wall of the cup, and of which some generally reach the centre. _Compound_ corals, again (fig.

44), consist of a greater or less number of structures similar in structure to the above, but united together in different ways into a common ma.s.s. _Simple_ corals, therefore, are the skeletons of _single_ and independent polypes; whilst _compound_ corals are the skeletons of a.s.semblages or colonies of similar polypes, living united with one another another as an organic community.