and the estuarine Coal-measures above, which characterizes the south of England, is gradually lost in going northwards, — beds of estuarine character gradually descending lower in the series in Yorkshire, as first shown by Professor Phillips, still lower in Northumberland, as described by Mr. G. Tate ; while in Scotland, as has been illustrated by Professor Geikie and other writers, the whole Carboniferous series from top to bottom consists of estuarine strata with subordinate marine beds intercalated.
The estuarine strata of Jurassic age in Scotland exhibit two different types of petrological character, which, though occasionally passing into one another by insensible gradations, are usually very distinct and easily recognizable. These we may distinguish as the argillaceous and arenaceous types. The fossils of these estuarine strata usually afford us comparatively little aid in determining the age of the several series : our knowledge of the succession of forms among the freshwater mollusca is far too limited for us to obtain much aid from this source ; and the few marine bands intercalated contain, as a rule, only specimens evidently dwarfed from unfavourable conditions, and scarcely ever belonging to highly characteristic forms like the Cephalopods and Echinoderms. The groups of marine strata, however, which alternate with the estuarine yield very fine and satisfactory faunas, by the study of which we are able to fix with great precision the limits of age of the latter.
The arenaceous type of the estuarine strata is characterized by beds of sandstone and grit, occasionally passing into conglomerates, and becoming in places somewhat calcareous. These alternate in some cases with subordinate beds of shale, and occasionally contain thin and imperfect seams of coal. The sandstone strata, which often attain to a great thickness, usually exhibit evidence of having been deposited under comparatively shallow- water conditions : false- bedding abounds ; surfaces with ripple-marks, worm-tracks, and other indications of the proximity of the shore frequently occur ; and the rock is usually crowded with fragments of carbonaceous matter. Large masses of wood, sometimes preserved as jet, and at other times presenting only hollow casts, abound in these sandstones ; and occasionally vertical plant-markings, like those of the Lower Oolite in England, are also found. Not unfrequently we observe a rock made up of alternations of laminae of sand and carbonaceous matter, so exactly resembling the strata seen in sections of old sand dunes, as strongly to suggest a similarity of origin. In all these sandstone strata molluscan remains are usually extremely rare ; but occasionally bands of obscure shells, almost always in the form of casts, are found ; these sometimes belong to marine, and at other times to freshwater genera.
The general resemblance of all these strata of the arenaceous type to those of the Lower Oolites of the Yorkshire coast is very striking ; and it was not unnatural that, at the early date at which they were first studied by Sir Roderick Murchison, this should be accepted as evidence of identity of age. Hence the whole of these
