X. THE CARBONIFEROUS SYSTEM, EMBRACING THE LOWER COAL-MEASURES, MOUNTAIN LIMESTONE, AND TRUE COAL-MEASURES. 89. IMMEDIATELY above the Old Red Sandstone, but clearly distinguishable from it by the abundance of their vegetable remains, occur the lower members of the CARBONIFEROUS SYSTEM. It is to this profusion of vegetable matter—the main solid element of which is carbon-that the system owes its name; a profusion which has formed seams of coal (coal being but a mass of mineralised vegetation), enters into the composition of all the black bituminous or coaly shales, and which stamps many of the sandstones and limestones with a carbonaceous aspect. As above indicated, the system is generally separable into three wellmarked groups-t -the lower coal-measures, or carboniferous slates; the mountain limestone; and the upper or true coal-measures. The student must not, however, suppose that these groups are everywhere present one above another in regular order. All that is affirmed by geology is, that these three groups are found in certain localities; and it is a rule of the science always to take as the type of a formation the fullest development that can be discovered. In some districts, as in the north of England, the carboniferous slates are absent, and the mountain limestone with its shales rests immediately on the old red sandstone; in other countries both the lower groups are absent, and the coal reposes on old crystalline rocks; while, on the other hand, in Ireland the carboniferous slates and mountain limestone are enormously developed, and the coal-measures very sparingly and partially so. Whatever portion of the system may be present, it is always easily recognised - the abundance and peculiarity of its fossil vegetables impressing it with features which, once seen, can never be mistaken for those of any other formation. Derived from the waste of all the preceding rocks-the granitic, metamorphic, silurian, and old red sandstone-the strata of the system necessarily present a great variety and complexity of composition, There are sandstones of every degree of purity, from thick beds composed of white quartz grains, to flaggy strata differing little from sandy shales; shales, from soft laminated clays to dark slaty flags, and from these to beds so bituminous that they are scarcely distinguishable from impure coals; and limestones, from sparkling saccharoid marbles to calcareous grits and shales. Besides these varieties of sandstones, clays, shales, and limestones, there occur, for the first time notably in the crust, seams of coal and bands of ironstone; and these also appearing in every degree of admixture, add still further to the complexity of the system. On the whole, the carboniferous strata, from first to last, may be said to be composed of frequent alternations of sandstones, shales, limestones, coals, and ironstones-and these in their respective groups we shall now consider. Lower Coal-Measures, or Carboniferous Slates. 90. This group is meant to embrace all the alternations of strata which lie between the old red sandstone and the mountain or carboniferous limestone. In some districts it is very scantily developed; in others, as in Ireland and Scotland, it attains a thickness of several thousand feet. In the south of Ireland it consists chiefly of dark slaty shales, grits, flaggy limestones, and thin seams of impure coal; and has, from the general slaty aspect of its strata, been termed the Carboniferous Slates. In Scotland, particularly in Fife and the Lothians, it has none of this slaty character, but consists essentially of thick-bedded white sandstones, dark bituminous shales, frequently imbedding bands of ironstone, thin seams of coal, and peculiar strata, either of shell-limestone or of argillaceous limestone, thought from its fossils to be of fresh-water or estuary origin. Unless in its fine white sandstones (the ordinary building-stone of Edinburgh and St Andrews), in its fine-grained estuary and shell limestones, and in the greater profusion of its shells and fishes, the lower group, as developed in Scotland, differs little in appearance from the upper group; hence the term Lower Coal-Measures generally applied to it in that country. 91. Looking at the lower coal-measures in the mass, there cannot be a doubt they were deposited under very different conditions from the old red sandstone beneath, and the mountain limestone above. Both these formations are eminently marine -the yellow sandstones being replete with true oceanic fishes, and the mountain limestone profusely charged with marine shells and corals. The lower coal-measures, on the other hand, have more of a fresh-water than of a salt-water aspect. Coralloid fossils are rarely, if ever, found in its strata; its shells are decidedly estuarine; its plants seem to have grown in marine marshes and delta jungles, and many of its fishes are large and of sauroid types. Lower Jaw and Dentition of Sauroid Fish-Rhizodus Hibbertii. Under these circumstances, we are justified in regarding it as a separate group-a group which, when more minutely investigated, will throw much important light on the earlier history of the period. 92. In its mineral composition and structure, this group bears evidence of frequent alternations of sediment, as if the rivers of transport were now charged with mud and vegetable debris, now with limy silt, and anon with sand and clay. There are no conglomerates as in the Old Red Sandstone, and from the laminated structure of most of the strata, they seem to have been deposited in tranquil waters. There are, however,, more frequent interstratifications of igneous rocks, as if the seas and estuaries of deposit had also been the seats of submarine volcanoes and craters of eruption. The iron which impregnated the waters of the Old Red period, and tinged with rusty red the whole of that system, now appears in the segregated form of thin layers and bands of ironstone. The frequent thin seams of coal point to a new exuberance of terrestrial vegetation, and indicate the existence of a genial climate and of dry lands of jungles where pines like the araucaria reared their gigantic trunks-of river-banks where tree-ferns waved their feathery fronds- and of estuarine and marine swamps where gigantic reed-like stems, equisetums, and other marsh vegetation, flourished in abundance. When we turn to the shell-limestones, and find them three or four feet in thickness, and entirely composed of mussel-like bivalves, we are instantly reminded of estuaries where these shell-fish lived in beds as do the mussel and other gregarious molluscs of the present day. Or if we examine the frequent remains of the fishes which are found in the shales and limestones, we have ample evidence of their predaceous habits, and are forcibly reminded of shallow seas and estuaries, where huge sauroid fishes were the tryantscavengers of the period. A few minute land-shells, and the skeletons of some small reptiles of the frog and lizard kinds, indicate the existence of a terrestrial fauna which becomes more abundant and varied in the higher groups of the system. Mountain or Carboniferous Limestones. 93. This group is one of the most distinct and unmistakable in the whole crust of the earth. Whether consisting of one thick reef-like bed of limestone, or of many beds with alternating shales and gritty sandstones, its peculiar corals, encrinites, and shells distinguish it at once from all other series of strata. In fact, it forms in the rocky crust a zone, so marked and peculiar, that it becomes a guiding-post, not only to the miner in the carboniferous system, but to the geologist in his researches among other strata. It has received the name of Mountain Limestone, because it is very generally found flanking or crowning the trap-hills that intervene between the Old Red and the Coal-Measures, where, from its hard and durable texture, it forms bold escarpments, as in the hills of Derbyshire, Yorkshire, Fife, and many parts of Ireland. It is also termed the Carboniferous Limestone, from its occurring in that system, and constituting one of its most remarkable features. 94. As already indicated, this group in some districts consists of a few thick beds of limestone, with subordinate layers of calcareous shale. In other localities the shales predominate, and the limestones occupy a subordinate place, alternating with the shales, thin seams of coal, and strata of gritty sandstones. Occasionally the limestone appears in one bold reef-like mass, of more than a hundred feet in thickness, separated by a few partings of shale, or rather layers of impure limestone. Whatever be the order of succession, it usually occurs as a dark sub-crystalline limestone, occasionally used as marble, but more frequently raised for mortar and agricultural purposes. Along with the other members of the group, it is often replete with the exuviæ of corals, encrinites, and shells, these fossils forming the curious ornamental markings on its polished surface. Besides being rent and dislocated like all other stratified rocks, it is further intersected by what are called joints or divisional planes (the "backs" and "cutters" of the quarryman)-these being fissures perpendicular to the lines of bedding, and causing the rock to break up in large tabular masses. These natural rents affording free passage to water, the mountain limestone is very often grooved and channeled; these channels, where the rock is thick, becoming caverns and grottoes of great extent and magnitude. It is to this percolation of water, charged with carbonic acid, that we owe not only these caverns, and the beautiful stalactites and stalagmites which adorn their roofs and floors, but also the numerous petrifying springs which abound in limestone districts. 95. The fossils of the limestone group are the usual coal-plants in the shales; and in the calcareous beds numerous varieties of corals, corallines, encrinites, shells, trilobites, and enamelscaled fishes, some of huge size and sauroid aspect. The whole of these fossils are highly indicative of marine conditions, and in 1, Syringopora; 2, Lithostrotion; 3, Aulopora; 4, Amplexus; 5, Clisiophyllum ; general the observer feels as little difficulty in accounting for the formation of the group, as he does in accounting for the origin of an existing coral-reef. Among the zoophytes the most characteristic are varieties of retepora and flustracea, whose net-like markings are found in almost every bed of calcareous shale; and |