five-toed bird-like imprints of which are numerous on some beds of sandstone. They are characteristically Mesozoic types of life. Another not less typically Mesozoic form, that of the Plesiosaurs,

likewise began in Triassic time; but it will be more particularly alluded to in the following chapter. The earliest known crocodiles

have been found in Triassic rocks ; some of the scutes or scales of one of these animals are shown in Fig. 169. But the most important advance in the fauna of the globe during the Triassic period was the first appearance of mammalian life. Detached teeth and lower jaws have been met with in the uppermost parts of the Triassic system, which have been identified as possessing structures like those of some of the marsupial animals of Australia (Microlestes, Fig. 170). It is interesting to know that the earliest representatives of the great class of the mammalia belonged to one of its lowest divisions. They were small creatures probably resembling the Ornithorhynchus and Echidna

The Triassic strata of the inland basins (England, Germany, France, etc.) have been subdivided into the following groups :—

Rhætic.

Red, green, and grey marls, black shales, sandstones, bone-beds, and in Germany sometimes thin seams of coal. Characteristic fossils are Cardium rhaticum, Avicula contorta, Pecten valoniensis, Pullestra arenicola, Acrodus, Ceratodus, Hybodus, Saurians, Microlestes.

Keuper or Upper
Trias.

Muschelkalk or
Middle Trias,

Bunter or Lower
Trias.

Red, grey, and green marls, with beds of rock-salt and gypsum.

Red sandstones and marls (England); grey sandstones and dark marls and clays, with thin seams of earthy coal (Germany).

Limestones and dolomites, with bands of anhydrite, gypsum, and rock-salt. The limestones are the great repository of the fossils. This subdivision is absent or only feebly represented in England.

Mottled red and green sandstones, marls, and sometimes pebble-beds.

The salt-beds of Cheshire have long been worked for commercial purposes. The lower bed is sometimes more than 100 feet thick; but the salt deposits of Germany are much more important. Thus at Sperenberg, 20 miles south of Berlin, a boring was put down through about 290 feet of gypsum, and then through upwards of 5000 feet of rock-salt, without reaching the bottom of the deposit.

The alternation of bands of rock-salt with thin layers of anhydrite or of gypsum no doubt marks successive periods of desiccation and inflow; in other words, each seam of sulphate of lime (which is the least soluble salt, and is therefore thrown down first) seems to indicate a renewed supply of salt water from outside, probably from the open sea, while the overlying rock-salt shows continued evaporation, during which the water became a concentrated solution and deposited a thicker layer of sodium chloride. Sometimes the concentration continued until still more soluble salts, such as chlorides of potassium and magnesium, were also eliminated. These phenomena are well displayed at the great salt-mines of Stassfurt, on the north flank of the Harz Mountains. The lowest rock there found is a mass of pure, solid, crystalline rock-salt of still unknown thickness, but which has been pierced for about 1000 feet. This rock is separated into layers, averaging about 3 inches in thickness, by partings of anhydrite 1 inch thick or less. If each of these "year rings," as the German miners call them, represented the deposit formed during the dry season of a single year, then the mass of 1000 feet would have taken more than 3000 years for its formation. there do not appear to be any good grounds for believing that each band marks one year's accumulation. Above the rock-salt lie valuable deposits of the more soluble salts, particularly chlorides of potassium and magnesium, with sulphates of lime and magnesia. The compound known as Carnallite (a double chloride of potassium

But

and magnesium) is now the chief source of the potash salts of

commerce.

In the Rhætic beds of England, one of the most interesting bands is the so-called "bone-bed" a thin layer of dark sandstone, charged with bones, teeth, and scales of fishes and saurians. A thin seam of limestone in the same group contains wings and wing-cases of insects.

The Trias of the Eastern Alps reaches a thickness of many thousand feet, and forms great ranges of mountains. The lower division runs throughout the Alps with considerable uniformity of character, so that it forms a useful platform from which to investigate the complicated geological structure of these mountains. The Upper Trias consists of several thousand feet of shales, marls, limestones, and dolomites, while the Rhætic group swells out into a great succession of limestones and dolomites. During the time when the Triassic sea stretched over the site of the Alps there were evidently considerable oscillations of level, and there likewise occurred extensive volcanic eruptions, whereby large masses of lavas and tuffs were ejected. These rocks now form conspicuous hills in the Tyrol.

Triassic rocks have been traced in Beloochistan, the salt range of the Punjab, Northern Kashmir, and Western Thibet. They cover a large area of North America, and have been recognised in Australia and New Zealand. Rocks which have been assigned to the same geological period occur in South Africa, and have there yielded a remarkable series of reptilian remains,

CHAPTER XXIII

JURASSIC

THE system which follows the Trias has received its name, JURASSIC, from the Jura Mountains, where it is well developed. It contains the record of a great series of geographical changes, which in Europe entirely effaced the inland basins and sandy wastes of the previous period, and during which sedimentary rocks were accumulated that now extend in a broad belt across England, from the coasts of Dorset to those of Yorkshire, cover an enormous area of France and Germany, and sweep along both sides of the Alps and the Apennines. These strata vary greatly in composition and thickness as they are traced from country to country. In one district they present a series of limestones which, as they are followed into another area, pass into shales or sandstones. The widespread uniformity of lithological character, so marked among the Palæozoic systems, gives place in the Mesozoic series to greater variety. Sandstones, shales, and limestones alternate more rapidly with each other, and are more local in their extent. They indicate greater vicissitudes in the process of deposition, more frequent alternations of sea and land, and not improbably greater differences of climate than in Palæozoic time.

The flora of the Jurassic period is marked by the same general characters as that of the Trias-ferns, equisetums, conifers, and cycads, being its distinguishing elements. Cycads now abound (Pterophyllum, Zamites, Cycadites, and many others, Fig. 171). Among the conifers are the remote ancestors of our “Puzzlemonkeys," introduced from Chili and now so common as ornamental garden shrubs (Araucaria imbricata), and of our pines and firs. This vegetation flourished luxuriantly over the area of Britain; on the site of Yorkshire it grew so densely as to give rise to thick peaty accumulations, which now form beds of coal. It went far northward, for its remains have been abundantly preserved in Spitzbergen, where numerous cycads have been found among them. These plants unquestionably grew and flourished within the Arctic Circle, so that, though the climates of the globe were already beginning to emerge from the greater uniformity of

FIG. 171. Jurassic Cycad (Cycadeoidea microphylla, &).

Palæozoic time, the Arctic regions still enjoyed a temperature like that of sub-tropical countries at the present time.

The animal world during the Jurassic period, if we may judge of it from its fossil remains, must have been much more varied alike on land and in the sea than during the previous ages of the earth's history. From the circumstances in which the strata were deposited, relics of the life of the land are frequently met with, besides abundant records of that of the sea. A characteristic feature of the period was the profusion of corals, which at different times spread over much of the site of modern Europe. They were no longer the rugose forms so distinctive of the Palæozoic

seas, but true reef-building astræids, belonging to the genera Isastrea, Thamnastraa, Montlivaltia, Thecosmilia, etc. (Fig. 172). Crinoids were still abundant, though less so than in the Carboniferous limestone sea; the old forms were now replaced by