The Student's Elements of Geology Part 45

The sandstones and clay of the Keuper resemble the deposits of estuaries and a shallow sea near the land, and afford, in the north-west of Germany, as in France and England, but a scanty representation of the marine life of that period. We might, however, have antic.i.p.ated, from its rich reptilian fauna, that the contemporaneous inhabitants of the sea of the Keuper period would be very numerous, should we ever have an opportunity of bringing their remains to light.

This, it is believed, has at length been accomplished, by the position now a.s.signed to certain Alpine rocks called the "St. Ca.s.sian beds," the true place of which in the series was until lately a subject of much doubt and discussion.

It has been proved that the Hallstadt beds on the northern flanks of the Austrian Alps correspond in age with the St. Ca.s.sian beds on their southern declivity, and the Austrian geologists, M. Suess of Vienna and others, have satisfied themselves that the Hallstadt formation is referable to the period of the Upper Trias. a.s.suming this conclusion to be correct, we become acquainted suddenly and unexpectedly with a rich marine fauna belonging to a period previously believed to be very barren of organic remains, because in England, France, and Northern Germany the upper Trias is chiefly represented by beds of fresh or brackish water origin.

(FIGURE 399. Scoliostoma, St. Ca.s.sian.)

(FIGURE 400. Platystoma Suessii, Hornes. From Hallstadt.)

(FIGURE 401. Koninckia Leonhardi, Wissmann.

a. Ventral view. Part of ventral valve removed to show the vascular impressions of dorsal valve.

b. Interior of dorsal valve, showing spiral processes restored.

c. Vertical section of both valves. Part shaded black showing place occupied by the animal, and the dorsal valve following the curve of the ventral.)

About 600 species of invertebrate fossils occur in the Hallstadt and St. Ca.s.sian beds, many of which are still undescribed; some of the Mollusca are of new and peculiar genera, as Scoliostoma, Figure 399, and Platystoma, Figure 400, among the Gasteropoda; and Koninckia, Figure 401, among the Brachiopoda.

TABLE 21.1 GENERA OF FOSSIL MOLLUSCA IN THE ST. Ca.s.sIAN AND HALLSTADT BEDS.

COLUMN 1: COMMON TO OLDER ROCKS.

Orthoceras.

Bactrites.

Macrocheilus.

Loxonema.

Holopella.

Murchisonia.

Porcellia.

Athyris.

Retzia.

Cyrtina.

Euomphalus.

COLUMN 2: CHARACTERISTIC TRIa.s.sIC GENERA.

Cerat.i.tes.

Cochloceras.

Choristoceras.

Rhabdoceras.

Aulacoceras.

Scoliostoma (reaches its maximum in the Trias, but pa.s.ses down to older rocks).

Naticella.

Platystoma.

Ptychostoma.

Euchrysalis.

Halobia.

Hornesia.

Amphiclina.

Koninckia.

Ca.s.sianella. (Reach their maximum in the Trias, but pa.s.s up to newer rocks.) Myophoria. (Reach their maximum in the Trias, but pa.s.s up to newer rocks.)

COLUMN 3: COMMON TO NEWER ROCKS.

Ammonites.

Chemnitzia.

Cerithium.

Monodonta.

Opis.

Sph.o.e.ra.

Cardita.

Myoconcha.

Hinnites.

Monotis.

Plicatula.

Pachyrisma.

Thecidium.

Table 21.1 of genera of marine sh.e.l.ls from the Hallstadt and St. Ca.s.sian beds, drawn up first on the joint authority of M. Suess and the late Dr. Woodward, and since corrected by Messrs. Etheridge and Tate, shows how many connecting links between the fauna of primary and secondary Palaeozoic and Mesozoic rocks are supplied by the St. Ca.s.sian and Hallstadt beds.

The first column marks the last appearance of several genera which are characteristic of Palaeozoic strata. The second shows those genera which are characteristic of the Upper Trias, either as peculiar to it, or, as in the three cases marked by asterisks, reaching their maximum of development at this era.

The third column marks the first appearance in Tria.s.sic rocks of genera destined to become more abundant in later ages.

It is only, however, when we contemplate the number of species by which each of the above-mentioned genera are represented that we comprehend the peculiarities of what is commonly called the St. Ca.s.sian fauna. Thus, for example, the Ammonite, which is not common to older rocks, is represented by no less than seventy-three species; whereas Loxonema, which is only known as common to older rocks, furnishes fifteen Tria.s.sic species. Cerithium, so abundant in tertiary strata, and which still lives, is represented by no less than fourteen species.

As the Orthoceras had never been met with in the marine Muschelkalk, much surprise was naturally felt that seven or eight species of the genus should appear in the Hallstadt beds, a.s.suming these last to belong to the Upper Trias.

Among these species are some of large dimensions, a.s.sociated with large Ammonites with foliated lobes, a form never seen before so low in the series, while the Orthoceras had never been seen so high.

On the whole, the rich marine fauna of Hallstadt and St. Ca.s.sian, now generally a.s.signed to the lowest members of the Upper Trias or Keuper, leads us to suspect that when the strata of the Tria.s.sic age are better known, especially those belonging to the period of the Bunter sandstone, the break between the Palaeozoic and Mesozoic Periods may be almost effaced. Indeed some geologists are not yet satisfied that the true position of the St. Ca.s.sian beds (containing so great an admixture of types, having at once both Mesozoic and Palaeozoic affinities) is made out, and doubt whether they have yet been clearly proved to be newer than the Muschelkalk.

MUSCHELKALK.

(FIGURE 402. Cerat.i.tes nodosus, Schloth. Muschelkalk, Germany. Side and front view, showing the denticulated outline of the septa dividing the chambers.)

(FIGURE 403. Gervilia (Avicula) socialis, Schloth. Characteristic sh.e.l.l of the Muschelkalk.)

The next member of the Trias in Germany, the Muschelkalk, which underlies the Keuper before described, consists chiefly of a compact greyish limestone, but includes beds of dolomite in many places, together with gypsum and rock-salt.

This limestone, a formation wholly unrepresented in England, abounds in fossil sh.e.l.ls, as the name implies. Among the Cephalopoda there are no belemnites, and no ammonites with foliated sutures, as in the Lias, and Oolite, and the Hallstadt beds; but we find instead a genus allied to the Ammonite, called Cerat.i.tes by de Haan, in which the descending lobes (Figure 402) terminate in a few small denticulations pointing inward. Among the bivalve crustacea, the Estheria minuta, Bronn (see Figure 390), is abundant, ranging through the Keuper, Muschelkalk, and Bunter-sandstein; and Gervillia socialis (Figure 403), having a similar range, is found in great numbers in the Muschelkalk of Germany, France, and Poland.

(FIGURE 404. Encrinus liliiformis, Schlott. Syn. E. moniliformis. Body, arms, and part of stem.

a. Section of stem. Muschelkalk.)

(FIGURE 405. Aspidura loricata, Aga.s.siz.

a. Upper side.

b. Lower side. Muschelkalk.)

(FIGURE 406. Palatal teeth of Placodus gigas. Muschelkalk.)

The abundance of the heads and stems of lily encrinites, Encrinus liliiformis (Figure 404), (or Encrinites moniliformis), shows the slow manner in which some beds of this limestone have been formed in clear sea-water. The star-fish called Aspidura loricata (Figure 405) is as yet peculiar to the Muschelkalk. In the same formation are found the skull and teeth of a reptile of the genus Placodus (see Figure 406), which was referred originally by Munster, and afterwards by Aga.s.siz, to the cla.s.s of fishes. But more perfect specimens enabled Professor Owen, in 1858, to show that this fossil animal was a Saurian reptile, which probably fed on sh.e.l.l-bearing mollusks, and used its short and flat teeth, so thickly coated with enamel, for pounding and crus.h.i.+ng the sh.e.l.ls.

BUNTER-SANDSTEIN.

(FIGURE 407. Voltzia heterophylla. (Syn. Voltzia brevifolia.) b. Portion of same magnified to show fructification. Sulzbad. Bunter-sandstein.)

The Bunter-sandstein consists of various-coloured sandstones, dolomites, and red clays, with some beds, especially in the Hartz, of calcareous pisolite or roe- stone, the whole sometimes attaining a thickness of more than 1000 feet. The sandstone of the Vosges is proved, by its fossils, to belong to this lowest member of the Tria.s.sic group. At Sulzbad (or Soultz-les-bains), near Strasburg, on the flanks of the Vosges, many plants have been obtained from the "bunter,"

especially conifers of the extinct genus Voltzia, of which the fructification has been preserved. (See Figure 407.) Out of thirty species of ferns, cycads, conifers, and other plants, enumerated by M. Ad. Brongniart, in 1849, as coming from the "Gres bigarre," or Bunter, not one is common to the Keuper.