The Student's Elements of Geology Part 61

HARLECH GRITS.

(FIGURE 573. Histioderma Hibernica, Kinahan. Oldhamia beds. Bray Head, Ireland.

1. Showing opening of burrow, and tube with wrinklings or crossing ridges, probably produced by a tentacled sea worm or annelid.

2. Lower and curved extremity of tube with five transverse lines.)

The sandstones of this period attain in the Longmynd hills a thickness of no less than 6000 feet without any interposition of volcanic matter; in some places in Merioneths.h.i.+re they are still thicker. Until recently these rocks possessed but a very scanty fauna.

With the exception of five species of annelids (see Figure 460) brought to light by Mr. Salter in Shrops.h.i.+re, and Dr. Kinahan in Wicklow, and an obscure crustacean form, Palaeopyge Ramsayi, they were supposed to be barren of organic remains. Now, however, through the labours of Mr. Hicks, they have yielded at St. David's a rich fauna of trilobites, brachiopods, phyllopods, and pteropods, showing, together with other fossils, a by no means low state of organisation at this early period. (British a.s.sociation Report 1868.) Already the fauna amounts to 20 species referred to 17 genera.

A new genus of trilobite called Plutonia Sedgwickii, not yet figured and described, has been met with in the Harlech grits. It is comparable in size to the large Paradoxides Davidis before mentioned, has well-developed eyes, and is covered all over with tubercles. In the same strata occur other genera of trilobites, namely, Conocoryphe, Paradoxides, Microdiscus, and the Pteropod Theca (Figure 568), all represented by species peculiar to the Harlech grits.

The sands of this formation are often rippled, and were evidently left dry at low tides, so that the surface was dried by the sun and made to shrink and present sun-cracks. There are also distinct impressions of rain-drops on many surfaces, like those in Figures 444 and 445.

LANBERIS SLATES.

(FIGURE 574. Oldhamia radiata, Forbes. Wicklow, Ireland.)

(FIGURE 575. Oldhamia antiqua, Forbes. Wicklow, Ireland.)

The slates of Llanberis and Penrhyn in Carnarvons.h.i.+re, with their a.s.sociated sandy strata, attain a great thickness, sometimes about 3000 feet. They are perhaps not more ancient than the Harlech and Barmouth beds last mentioned, for they may represent the deposits of fine mud thrown down in the same sea, on the borders of which the sands above-mentioned were acc.u.mulating. In some of these slaty rocks in Ireland, immediately opposite Anglesea and Carnarvon, two species of fossils have been found, to which the late Professor E. Forbes gave the name of Oldhamia. The nature of these organisms is still a matter of discussion among naturalists.

CAMBRIAN ROCKS OF BOHEMIA (PRIMORDIAL ZONE OF BARRANDE).

In the year 1846, as before stated, M. Joachim Barrande, after ten years'

exploration of Bohemia, and after collecting more than a thousand species of fossils, had ascertained the existence in that country of three distinct faunas below the Devonian. To his first fauna, which was older than any then known in this country, he gave the name of Etage C; his two first stages A and B consisting of crystalline and metamorphic rocks and unfossiliferous schists.

This Etage C or primordial zone proved afterwards to be the equivalent of those subdivisions of the Cambrian groups which have been above described under the names of Menevian and Lingula Flags. The second fauna tallies with Murchison's Lower Silurian, as originally defined by him when no fossils had been discovered below the Stiper-Stones. The third fauna agrees with the Upper Silurian of the same author. Barrande, without government a.s.sistance, had undertaken single- handed the geological survey of Bohemia, the fossils previously obtained from that country having scarcely exceeded 20 in number, whereas he had already acquired, in 1850, no less than 1100 species, namely, 250 crustaceans (chiefly Trilobites), 250 Cephalopods, 160 gasteropods and pteropods, 130 acephalous mollusks, 210 brachiopods, and 110 corals and other fossils. These numbers have since been almost doubled by subsequent investigations in the same country.

(Figures 576 to 580. Fossils of the lowest Fossiliferous Beds in Bohemia, or "Primordial Zone" of Barrande.

(FIGURE 576. Paradoxides Bohemicus, Barr. About one-half natural size.)

(FIGURE 577. Conocoryphe striata. Syn. Conocephalus striatus, Emmrich. One-half natural size. Ginetz and Skrey.)

(FIGURE 578. Agnostus integer, Beyrich. Natural size and magnified.)

(FIGURE 579. Agnostus Rex, Barr. Natural size, Skrey.)

(FIGURE 580. Sao hirsuta, Barrande, in its various stages of growth. The small lines beneath indicate the true size. In the youngest state, a, no segments are visible; as the metamorphosis progresses, b, c, the body segments begin to be developed: in the stage d the eyes are introduced, but the facial sutures are not completed; at e the full-grown animal, half its true size, is shown.))

In the primordial zone C, he discovered trilobites of the genera Paradoxides, Conocoryphe, Ellipsocephalus, Sao, Arionellus, Hydrocephalus, and Agnostus. M.

Barrande pointed out that these primordial trilobites have a peculiar facies of their own dependent on the multiplication of their thoracic segments and the diminution of their caudal s.h.i.+eld or pygidium.

One of the "primordial" or Upper Cambrian Trilobites of the genus Sao, a form not found as yet elsewhere in the world, afforded M. Barrande a fine ill.u.s.tration of the metamorphosis of these creatures, for he traced them through no less than twenty stages of their development. A few of these changes have been selected for representation in Figure 580, that the reader may learn the gradual manner in which different segments of the body and the eyes make their appearance.

In Bohemia the primordial fauna of Barrande derived its importance exclusively from its numerous and peculiar trilobites. Besides these, however, the same ancient schists have yielded two genera of brachiopods, Orthis and Orbicula, a Pteropod of the genus Theca, and four echinoderms of the cystidean family.

CAMBRIAN OF SWEDEN AND NORWAY.

The Cambrian beds of Wales are represented in Sweden by strata the fossils of which have been described by a most able naturalist, M. Angelin, in his "Palaeontologica Suecica" (1852-4). The "alum-schists," as they are called in Sweden, are horizontal argillaceous rocks which underlie conformably certain Lower Silurian strata in the mountain called Kinnekulle, south of the great Wener Lake in Sweden. These schists contain trilobites belonging to the genera Paradoxides, Olenus, Agnostus, and others, some of which present rudimentary forms, like the genus last mentioned, without eyes, and with the body segments scarcely developed, and others, again, have the number of segments excessively multiplied, as in Paradoxides. Such peculiarities agree with the characters of the crustaceans met with in the Cambrian strata of Wales; and Dr. Torell has recently found in Sweden the Paradoxides Hicksii, a well-known Lower Cambrian fossil.

At the base of the Cambrian strata in Sweden, which in the neighbourhood of Lake Wener are perfectly horizontal, lie ripple-marked quartzose sandstones with worm-tracks and annelid borings, like some of those found in the Harlech grits of the Longmynd. Among these are some which have been referred doubtfully to plants. These sandstones have been called in Sweden "fucoid sandstones." The whole thickness of the Cambrian rocks of Sweden does not exceed 300 feet from the equivalents of the Tremadoc beds to these sandstones, which last seem to correspond with the Longmynd, and are regarded by Torell as older than any fossiliferous primordial rocks in Bohemia.

CAMBRIAN OF THE UNITED STATES AND CANADA (POTSDAM SANDSTONE).

(FIGURE 581. Dikelocephalus Minnesotensis. Dale Owen. One-third diameter. A large crustacean of the Olenoid group. Potsdam sandstone. Falls of St. Croix, on the Upper Mississippi.)

This formation, as we learn from Sir W. Logan, is 700 feet thick in Canada; the upper part consists of sandstone containing fucoids, and perforated by small vertical holes, which are very characteristic of the rock, and appear to have been made by annelids (Scolithus linearis). The lower portion is a conglomerate with quartz pebbles. I have seen the Potsdam sandstone on the banks of the St.

Lawrence, and on the borders of Lake Champlain, where, as at Keesville, it is a white quartzose fine-grained grit, almost pa.s.sing into quartzite. It is divided into horizontal ripple-marked beds, very like those of the Lingula Flags of Britain, and replete with a small round-shaped Obolella, in such numbers as to divide the rock into parallel planes, in the same manner as do the scales of mica in some micaceous sandstones. Among the sh.e.l.ls of this formation in Wisconsin are species of Lingula and Orthis, and several trilobites of the primordial genus Dikelocephalus (Figure 581). On the banks of the St. Lawrence, near Beauharnois and elsewhere, many fossil footprints have been observed on the surface of the rippled layers. They are supposed by Professor Owen to be the trails of more than one species of articulate animal, probably allied to the King Crab, or Limulus.

Recent investigations by the naturalists of the Canadian survey have rendered it certain that below the level of the Potsdam Sandstone there are slates and schists extending from New York to Newfoundland, occupied by a series of trilobitic forms similar in genera, though not in species, to those found in the European Upper Cambrian strata.

HURONIAN SERIES.

Next below the Upper Cambrian occur strata called the Huronian by Sir W. Logan, which are of vast thickness, consisting chiefly of quartzite, with great ma.s.ses of greenish chloritic slate, which sometimes include pebbles of crystalline rocks derived from the Laurentian formation, next to be described. Limestones are rare in this series, but one band of 300 feet in thickness has been traced for considerable distances to the north of Lake Huron. Beds of greenstone are intercalated conformably with the quartzose and argillaceous members of this series. No organic remains have yet been found in any of the beds, which are about 18,000 feet thick, and rest unconformably on the Laurentian rocks.

LAURENTIAN GROUP.

In the course of the geological survey carried on under the direction of Sir W.E. Logan, it has been shown that, northward of the river St. Lawrence, there is a vast series of crystalline rocks of gneiss, mica-schist, quartzite, and limestone, more than 30,000 feet in thickness, which have been called Laurentian, and which are already known to occupy an area of about 200,000 square miles. They are not only more ancient than the fossiliferous Cambrian formations above described, but are older than the Huronian last mentioned, and had undergone great disturbing movements before the Potsdam sandstone and the other "primordial" or Cambrian rocks were formed. The older half of this Laurentian series is unconformable to the newer portion of the same.

UPPER LAURENTIAN OR LABRADOR SERIES.

The Upper Group, more than 10,000 feet thick, consists of stratified crystalline rocks in which no organic remains have yet been found. They consist in great part of feldspars, which vary in composition from anorthite to andesine, or from those kinds in which there is less than one per cent of potash and soda to those in which there is more than seven per cent of these alkalies, the soda preponderating greatly. These feldsparites sometimes form mountain ma.s.ses almost without any admixture of other minerals; but at other times they include augite, which pa.s.ses into hypersthene. They are often granitoid in structure. One of the varieties is the same as the apolescent labradorite rock of Labrador. The Adirondack Mountains in the State of New York are referred to the same series, and it is conjectured that the hypersthene rocks of Skye, which resemble this formation in mineral character, may be of the same geological age.

LOWER LAURENTIAN.

This series, about 20,000 feet in thickness, is, as before stated, unconformable to that last mentioned; it consists in great part of gneiss of a reddish tint with orthoclase feldspar. Beds of nearly pure quartz, from 400 to 600 feet thick, occur in some places. Hornblendic and micaceous schists are often interstratified, and beds of limestone, usually crystalline. Beds of plumbago also occur. That this pure carbon may have been of organic origin before metamorphism has naturally been conjectured.

(FIGURES 582 and 583. Eozoon Canadense, Daw. (after Carpenter). Oldest known organic body.

(FIGURE 582. Eozoon Canadense, Daw. (after Carpenter). Oldest known organic body.

a. Chambers of lower tier communicating at +, and separated from adjoining chambers at o by an intervening septum, traversed by pa.s.sages.

b. Chambers of an upper tier.

c. Walls of the chambers traversed by fine tubules. (These tubules pa.s.s with uniform parallelism from the inner to the outer surface, opening at regular distances from each other.) d. Intermediate skeleton, composed of h.o.m.ogeneous sh.e.l.l substance, traversed by f. Stoloniferous pa.s.sages connecting the chambers of the two tiers.

e. Ca.n.a.l system in intermediate skeleton, showing the arborescent saceodic prolongations.

(Figure 583 shows these bodies in a decalcified state.))

(FIGURE 583. Eozoon Canadense, Daw. (after Carpenter). Oldest known organic body.

Decalcified portion of natural rock, showing Ca.n.a.l SYSTEM and the several layers; the acuteness of the planes prevents more than one or two parallel tiers being observed. Natural size.))

There are several of these limestones which have been traced to great distances, and one of them is from 700 to 1500 feet thick. In the most ma.s.sive of them Sir W. Logan observed, in 1859, what he considered to be an organic body much resembling the Silurian fossil called Stromatopora rugosa. It had been obtained the year before by Mr. J. MacMullen at the Grand Calumet, on the river Ottawa.

This fossil was examined in 1864 by Dr. Dawson of Montreal, who detected in it, by aid of the microscope, the distinct structure of a Rhizopod or Foraminifer.

Dr. Carpenter and Professor T. Rupert Jones have since confirmed this opinion, comparing the structure to that of the well-known nummulite. It appears to have grown one layer over another, and to have formed reefs of limestone as do the living coral-building polyp animals. Parts of the original skeleton, consisting of carbonate of lime, are still preserved; while certain inter-s.p.a.ces in the calcareous fossil have been filled up with serpentine and white augite. On this oldest of known organic remains Dr. Dawson has conferred the name of Eozoon Canadense (see Figures 582, 583); its antiquity is such that the distance of time which separated it from the Upper Cambrian period, or that of the Potsdam sandstone, may, says Sir W. Logan, be equal to the time which elapsed between the Potsdam sandstone and the nummulitic limestones of the Tertiary period. The Laurentian and Huronian rocks united are about 50,000 feet in thickness, and the Lower Laurentian was disturbed before the newer series was deposited. We may naturally expect the other proofs of unconformability will hereafter be detected at more than one point in so vast a succession of strata.

The mineral character of the Upper Laurentian differs, as we have seen, from that of the Lower, and the pebbles of gneiss in the Huronian conglomerates are thought to prove that the Laurentian strata were already in a metamorphic state before they were broken up to supply materials for the Huronian. Even if we had not discovered the Eozoon, we might fairly have inferred from a.n.a.logy that as the quartzites were once beds of sand, and the gneiss and mica-schist derived from shales and argillaceous sandstones, so the calcareous ma.s.ses, from 400 to 1000 feet and more in thickness, were originally of organic origin. This is now generally believed to have been the case with the Silurian, Devonian, Carboniferous, Oolitic, and Cretaceous limestones and those nummulitic rocks of tertiary date which bear the closest affinity to the Eozoon reefs of the Lower Laurentian. The oldest stratified rock in Scotland is that called by Sir R.

Murchison "the fundamental gneiss," which is found in the north-west of Ross- s.h.i.+re, and in Sutherlands.h.i.+re (see Figure 82), and forms the whole of the adjoining island of Lewis, in the Hebrides. It has a strike from north-west to south-east, nearly at right angles to the metamorphic strata of the Grampians.

On this Laurentian gneiss, in parts of the western Highlands, the Lower Cambrian and various metamorphic rocks rest unconformably. It seems highly probable that this ancient gneiss of Scotland may correspond in date with part of the great Laurentian group of North America.

CHAPTER XXVIII.