Fossils of the lowest Fossiliferous Beds in Bohemia, or
“Primordial Zone” of Barrande.
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 illustration 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 “Palæontologica 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).—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 passing 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 shells of this formation in Wisconsin are species of Lingula and Orthis, and several trilobites of the primordial genus Dikelocephalus (Fig. 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 masses 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.