salts, particularly chlorides of potassium and magnesium, with sulphates of lime and magnesia. The compound known as Carnallite (a double chloride of potassium 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 XXII.

JURASSIC.

THE Jurassic system which follows the Trias has received its name 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, 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

FIG. 171. -Jurassic Cycad (Cycadeoidea microphylla, ৳).

ancestors of our “Puzzle-monkeys," 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 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 Isastræa, Thamnastrea, Montlivaltia, Thecosmilia, etc. (Fig. 172).

the Cornbrash.

Crinoids were still abundant, though less so than in the Carboniferous limestone sea; the old forms were now replaced by others, among which the most conspicuous was the Pentacrinite (Fig. 173) - a genus still living in our present seas. Sea-urchins swarmed on some parts of the sea-floor; among their more frequent genera are Cidaris (Fig. 174), Diadema, Hemicidaris, Acrosalenia, Glyptichus, Pygaster. Of the contrasts between the Mesozoic and Palæozoic faunas one of the most marked is to be found among the brachiopods. Except the persistent inarticulate types which have lived on from Cambrian time to the

FIG. 173. Jurassic Crinoid (Pentacrinus fasciculosus, ).

present day (Crania, Lingula, Discina), the numerous and varied forms which played so important a part in the life of the Palæozoic seas died out almost entirely at the close of the Palæozoic period. The ancient Spirifers and Leptænids lingered on until the Jurassic period, and then disappeared. On the other hand, the genera Rhynchonella and Terebratula, which occupied a subordinate place in earlier ages, now became the chief representatives of the