The Rhætic beds of Europe were, as a whole, formed under very different conditions in different areas. The thickness of the strata and the large and well-developed fauna (chiefly Mollusca) indicate that the Rhætic strata of Lombardy, and other parts of the south and east of Europe, were deposited in a broad open ocean. On the other hand, the comparatively thin beds of this age in England and north-western Europe, the fauna of which, besides being poor in genera and species, consists of small and dwarfed forms, point to the conclusion that they were in great part deposited in shallow seas and in estuaries, or in lagoons, or in occasional salt lakes, under conditions which lasted for a long period.[56]
In consequence of the importance they assume in Lombardy (the ancient Rhætia), the name “Rhætic Beds” has been given to these strata by Mr. Charles Moore; Dr. Thomas Wright has proposed the designation “Avicula Contorta Zone,” from the plentiful occurrence of that shell in the black shales forming the well-marked middle zone, and which is everywhere present where this group of beds is found; Jules Martin and others have proposed the term “Infra-lias,” or “Infra-liassic strata;” while the name “Penarth Beds” has been assigned to these deposits in this country by Mr. H. W. Bristow, at the suggestion of Sir Roderick Murchison, in consequence of their conspicuous appearance and well-exposed sections in the bold headlands and cliffs of that locality, in the British Channel, west of Cardiff.
A fuller description of these beds will be found in the Reports of the Bath Meeting of the British Association (1864), by Mr. Bristow; also in communications to the Geological Magazine, for 1864, by MM. Bristow and Dawkins;[57] in papers read before the Geological Society by Dr. Thomas Wright,[58] Mr. Charles Moore,[59] and Mr. Ralph Tate,[60] as printed in their Quarterly Journal; and by Mr. Etheridge, in the Transactions of the Cotteswold Natural History Club for 1865-66. The limits of the Penarth Beds have also been lately accurately laid down by Mr. Bristow in the map of the Geological Survey over the district comprised between Bath, Bristol, and the Severn; and elaborately detailed typical sections of most of the localities in England, where these beds occur, have been constructed by MM. Bristow, Etheridge, and Woodward, of the Geological Survey of Great Britain, which, when published, will greatly add to our knowledge of this remarkable and interesting series of deposits.
JURASSIC PERIOD.
This period, one of the most important in the physical history of the globe, has received its name from the Jura mountains in France, the Jura range being composed of the rocks deposited in the seas of the period. In the term Jurassic, the formations designated as the “Oolite” and “Lias” are included, both being found in the Jura mountains. The Jurassic period presents a very striking assemblage of characteristics, both in its vegetation and in the animal remains which belong to it; many genera of animals existing in the preceding age have disappeared, new genera have replaced them, comprising a very specially organised group, containing not less than 4,000 species.
The Jurassic period is sub-divided into two sub-periods: those of the Lias and the Oolite.
The Lias
is an English provincial name given to an argillaceous limestone, which, with marl and clay, forms the base of the Jurassic formation, and passes almost imperceptibly into the Lower Oolite in some places, where the Marlstone of the Lias partakes of the mineral character, as well as the fossil remains of the Lower Oolite; and it is sometimes treated of as belonging to that formation. “Nevertheless, the Lias may be traced throughout a great part of Europe as a separate and independent group, of considerable thickness, varying from 500 to 1,000 feet, containing many peculiar fossils, and having a very uniform lithological aspect.”[61] The rocks which represent the Liassic period form the base of the Jurassic system, and have a mean thickness of about 1,200 feet. In the inferior part we find argillaceous sandstones, which are called the sandstones of the Lias, and comprehend the greater part of the Quadersandstein, or building-stone of the Germans, above which comes compact limestone, argillaceous, bluish, and yellowish; finally, the formation terminates in the marlstones which are sometimes sandy, and occasionally bituminous.
The Lias, in England, is generally in three groups: 1, the upper, clays and shales, underlying sands; 2, the middle, lias or marlstone; and 3, the lower, clays and limestone; but these have been again sub-divided—the last into six zones, each marked by its own peculiar species of Ammonites; the second into three zones; the third consists of clay, shale, and argillaceous limestone. For the purposes of description we shall, therefore, divide the Lias into these three groups:—