which were doubtless much steeper and higher then than they are now.

It will be seen from the map (Plate VI.) that I have supposed the Scottish Triassic deposits to lie on the sites of two smaller and separate lacustrine areas. It might, of course, be argued that the western basin was only an arm of the Anglo-Hibernian lake, and that it was also connected with the north-eastern basin by a narrow channel along the site of the Great Glen; but the absence of rock-salt and gypsum in the Scotch Trias, and the presence of bivalve shells resembling Cyrena at Ardtornish, are facts which make it probable that the water of the Scotch lakes was fresh, and not salt, and consequently that the connection between them and the salt lake was by means of a river channel.

The absence of the Rhætic beds and the estuarine character of the Lower Lias in Sutherland are also facts which confirm the view that the Scotch lakes were at a somewhat higher level than the great salt lake; and the contrast between the marine and estuarine character of the Lower Lias of the two districts seems to indicate that the level of the eastern was higher than that of the western. Hence we seem justified in concluding that in Triassic times the eastern discharged itself into the western, and that the overflow of the latter was conducted by a river into the western arm of the great salt lake. We may, in fact, regard the Scotch lakes as the mountain-fed reservoirs of the Anglo-Hibernian lake.

THE

CHAPTER IX.

JURASSIC PERIOD.

HE Jurassic strata, comprising the Lias and the Oolitic series, succeed the Triassic marls with complete conformity, the passage from one system to the other being through a group of grey marls, shales, and limestones, which are known as the Rhætic or Penarth Beds. The Jurassic system is divisible into three rock groups or series.

The Lower or Liassic series is essentially a clay formation, with occasional bands of limestone, sand, and ironstone of variable thickness.

The Middle Jurassic series consists mainly of limestones, with only subordinate bands of sand and clay.

The Upper Jurassic, again, is an argillaceous series, the limestones being discontinuous, and sometimes absent or replaced by clays.

The most persistent formations are the three great clays: the Lower Lias, the Oxford, and the Kimeridge Clays. These range all across England, and form broad tracts of low-lying land; while the intermediate limestones, where they are well developed, form long ridges, with escarpments facing the west or north-west, in consequence of the prevalent easterly dip.

§ 1. Stratigraphical Evidence.

Rhætic and Lias.-The Rhætic Beds, though in Britain they are quite a subordinate division, are of special interest,

because they mark the epoch when the great Triassic lake was first invaded by the sea. At, or near the base of the Rhætic shales, there is usually a layer of shaly sandstone, which contains phosphatic nodules and is crowded with the remains of fish and small reptiles, and sometimes there are several such layers. It would appear as if the sudden irruption of the sea-water was prejudicial to the inhabitants of the Triassic lake, so that most of them died, and their bones, scales, and teeth were drifted into layers on the sea-floor.

The Rhætic Beds have been found everywhere in England where the junction of the Trias and Lias is exposed, and they occur in the north-east of Ireland, but are not known in Scotland. It is true that on the western Scottish coasts the base of the Lias is nowhere clearly exposed, but Professor Judd thinks that no representative of the Rhætic shales exists there. These beds, therefore, so far as our present knowledge enables us to judge, were confined to the area of the Anglo-Hibernian lake. On the borders of this area, as on the Mendip Hills and in Glamorganshire, the shales and limestones are sometimes replaced by sands and sandstones.

The Lias has a wider extension; its thickness is often more than 1,000 feet, and it must have overlapped the Trias more or less in every direction, though the actual extent of the Lower Lias may not have been very much greater than that of the Trias, because the western coast of the Triassic lake was in many places very steep. The broad outcrop of the Lias stretches across England from Dorset to Yorkshire, and outlying tracts occur in Staffordshire, Shropshire, and Cumberland, and in the north-east of Ireland. They occur also on both sides of the Scottish Highlands.

The thick clays and shales of this series indicate a sea into which many rivers discharged a constant supply of

muddy material derived from the waste of the surrounding land. The shaly layers, which are familiar to us under the name of Lias, are evidently such as were formed in the more central and deeper parts of the sea, and there are only a few localities where littoral beds of this age have been preserved; but of these it is desirable to give some special account.

Shore-beds of Lower Liassic age are known in four districts, viz., the Mendip district, Glamorganshire, the Inner Hebrides, and eastern Sutherland. In the Mendip district, near Shepton-Mallet, the ordinary clays and thin limestones pass into massive white limestones, associated with conglomerates composed of Carboniferous Limestone and chert. Again, on the northern flank of the hills near Chewton-Mendip, and Harptree, there is a compact cherty deposit, containing Lower Lias fossils, and resting indifferently on Old Red Sandstone, Carboniferous Limestone, and Dolomitic Conglomerate.'

Similar deposits occur in Glamorganshire, near Bridgend, Sutton, Brocastle, and Cowbridge, the basal beds being hard, cherty, conglomeratic limestones, and passing up into massive limestones full of fossils. The Sutton stone is a soft white limestone, and in it corals are particularly abundant.

In the west of Scotland the lower part of the Lias resembles that of South Wales, consisting of hard limestones alternating with calcareous and conglomeratic sandstones. Above these are shelly limestones and shales. On the east coast these beds are represented by estuarine deposits. At the base are coarse sandstones and conglomerates, with pebbles derived from the Lias, and these pass up into a series of sandstones and shales, with thin layers of clay and coal, the whole attaining a thickness of between 400 and 500 feet.

1 Woodward, “Geology of England,” second edition, p. 265.

It is rather remarkable that in three of these districts the shore-beds should be chiefly limestones, and we must infer that in these places, at least, very little detritus of any kind was carried in from the land at the beginning of the Liassic period. That this should be the case round the Mendip island is quite natural, but that limestones should be formed on the margin of the western inlet between Wales and Devon requires explanation; possibly this is to be found in the supposition that freshwater lakes existed in the country to the west, and that these for a time arrested and detained the mechanical detritus brought down by the rivers, leaving only the calcareous matter in solution to be carried on to the sea by the effluent stream. This, when added to the lime derived from the waste of the Carboniferous Limestone along the shore, was more than the sea-water could hold in solution, and the formation of limestones was the result. In the Scotch case, we may suppose that the Sutherland basin was the lake or lagoon which received the detritus, and thus allowed the formation of limestones in the western gulf.

The only shore-beds of Middle Liassic age preserved to us are those on the Scottish coast, where the Scalpa beds of Professor Judd,' in the islands of Scalpa, Skye, and Raasay, consist of calcareous sandstones, 200 feet thick, containing the fossils of the English Marlstone. In Mull they are represented by soft greenish sandstone with few fossils; and similar beds seem to have been formed in the eastern basin, blocks of them occurring in the Boulder clays of Elgin and Moray.

No marginal deposits of Upper Liassic age are known, but it may be noticed that round the Mendip Hills the water was very shallow throughout the Liassic period, the whole series being in some places represented by only

1 "Quart. Journ. Geol. Soc.," vol. xxxiv. p. 710.