Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/357

palzeontological break can generally be traced in the Old Red Sandstone, dividing it into two completely distinct series—a Lower, which graduates downward into the Upper Silurian, and an Upper, which passes upward into the base of the Carboniferous system.

As a whole, the Old Red Sandstone, where its strata are really red, is like other masses of red deposits, singularly barren of organic remains. The physical conditions under which the precipitation of iron oxide took place were evidently unfavourable for the development of animal life in the same waters. Professor Ramsay has connected the occurrence of such red formations with the existence of salt lakes, from the bitter waters of which not only iron-oxide but often rock-salt, magnesian limestone, and gypsum were thrown down. He points also to the presence of land-plants, footprints of amphibia, and other indications of terrestrial surfaces, while truly marine organisms are either found in a stunted condition or are absent altogether. 'here the strata of the Old Red Sandstone, losing their red colour and ferruginous character, assume grey or yellow tints and pass into a calcareous or argillaceous condition, they not infrequently become fossiliferous. At the same time it is not unworthy of remark that some of the red conglomerates, which might be supposed little likely to contain organic remains, are occasionally found to be full of detached scales, plates, and bones of fishes.

Along the border of the Silurian region from Shropshire into South Wales the uppermost parts of the Silurian system graduate into a mass of red strata not less than 10,000 feet thick, which in turn pass 11p conformably into the base of the Carboniferous system. This vast accumulation of red rocks, termed the Old Red Sandstone. consists in its lower portions of red and green shales and flagstones, with some white sandstones and thin cornstones; in the central and chief division, of red and green spotted sandy marls and clays, with red sandstones and cornstones; in the higher parts of grey, red, chocolate—coloured, and yellow sandstones, with bands of conglomerate. No unconformability has yet been detected in any part of this series of rocks, though, from the observations of De la Beche, it may be suspected that the higher strata which graduate upward into the Carboniferous formations are separated from the underlying portions of the Old Red Sandstone by a distinct discordance.

Although, as a whole, barren of organic remains, these red rocks have here and there, more particularly in the calcareous zones, yielded fragments of fishes and crustaceans. In their lower and central portions remains of the ganoids Cephalaspis, Didymaspis, Scaphasis, Pteraspis, and Cyathaspis have been found, together with crustaceans of the genera Stylonurus, Pterygotus, and Prearturus, and obscure traces of plants. The upper yellow and red sandstones contain none of the cephalaspid fishes, which are there replaced by Ptericthys and Holoptychius, associated with distinct impressions of land—plants. In some of the higher parts of the Old Red Sandstone of South Wales and Shropshire, Serpula and Condaria occur; but these are exceptional cases, and point to the advent of the Carboniferous marine fauna, which doubtless existed outside the British area before it spread over the Old Red Sandstone basins. It is in Scotland that the Old Red Sandstone shows the most complete and varied development, alike in physical structure and in organic contents. Throughout that country the system is found everywhere to present a division into two well-marked groups of strata, separated from each other by a strong unconformability and a complete break in the succession of organic remains. It occurs in distinct areas which appear to mark the site of separate basins of deposit. One of these occupies the central valley between the base of the Highland mountains and the uplands of the southern counties. On the north-east it is cut off by the present coast-line from Stonehaven to the mouth of the Tay. On the south—west it ranges by the island of Arran across St George’s Channel into Ireland, where it runs almost to the western sea-board, flanked on the north, as in Scotland, by hills of crystalline rocks and on the south chiefly by a Lower Silurian belt. Another distinct and still larger basin lies on the north side of the Highlands, but only a portion of it comes within the present area of Scotland. It skirts the slopes of the mountains along the Moray Firth and the cast of Ross and Sutherland, and stretches through Caithness and the Orkney Islands as far as the south of the Shetland group. It may possibly have been at one time continued as far as the Sognefjord and Dalsfjord in Norway, where red conglomerates like those of the north of Scotland occur. There is even reason to infer that it may have ranged eastwards into Russia, for some of its most characteristic organisms are found also among the red sandstones of that country. A third minor area of deposit lay on the south side of the southern uplands over the east of Berwickshire and the north of Northumberland, including the area of the Cheviot Hills. A fourth occupied a basin on the flanks of the south-west Highlands, which is now partly marked by the terraced hills of Lorne. There is sufficient diversity of lithological and palzeontological characters to show that these several areas were distinct basins, separated both from each other and from the sea.

In the central basin of Scotland between the Highlands and the southern uplands, the twofold division of the Old Red Sandstone is typically seen. The lower series of deposits attains a maximum depth of upwards of 20,000 feet. In Lanarkshire it is found to pass down conformably into the Upper Silurian rocks; elsewhere its base is concealed by later formations, or by the unconformability with which different horizons rest upon the older rocks. It is covered unconformably by every formation younger than itself. It consists of reddish—brown or chocolate-coloured, grey, and yellow sandstones, red shales, grey flagstones, coarse conglomerates, and occasional bands of limestone and cornstone. The grey flagstones and thin grey and olive shales and “calmstones” are almost confined to Forfarshire, in the north-east part of the basin, and are known as the Arbroath flags. One of the most marked lithological features in this central Scottish basin is the prodigious masses of interbedded volcanic rocks. These, consisting of porphyrite—lavas, felsites, and tuffs, attain a thickness of more than 6000 feet, and form important chains of bills, as in the Pentland, Ochil, and Sidlaw ranges. They lie several thousand feet above the base of the system, and are regularly interstratified here and there with bands of the ordinary sedimentary strata. They point to the outburst. of numerous volcanic vents along the lake or inland sea in which the Lower Old Red Sandstone of central Scotland was laid down; and their disposition shows that these vents ranged themselves in lines or linear groups parallel with the general trend of the great central valley. The fact that the igneous rocks are succeeded by thousands of feet of sand- stones, shales, and conglomerates, without any intercalation of lava or tuff, proves that the volcanic episode in the history of the lake came to a close long before the lake itself disappeared.

As a rule the deposits of this lake are singularly unfossiliferous, though some portions of them, particularly in the Forfarshire flagstone group, have proved rich in fish remains. In Lanarkshire about 5000 feet above the base of the system a thin hand of shale occurs, containing a graptolite, Spirorbis Lewisii, and Orthoceras dimidiatum,— undoubtedly Upper Silurian forms. This interesting act serves to indicate that, though geographical changes had