By referring to the diagram of this coal-field, given above at p. 196, the reader will notice that the series is divisible into three groups:--1_st_, and undermost, a considerable depth of coal bearing strata known as the _edge series_, because they lie along the western limits of the coal-basin at a high angle, and sometimes even on edge; 2_d_, A great thickness of sandstones nearly barren of coal, but containing at least three beds of limestone this may be termed the Roslyn sandstone group; 3_d_, and highest, another series of coal-bearing strata, commonly called the _flat coals_, because they occupy the centre of the basin where the beds repose at a low angle, and are in places quite flat. It will be convenient to keep in mind this three-fold division, for it will point us to some important changes in the ancient conditions of this coal-field. The edge series, which forms the lowest, and of course oldest of the above groups, averages from 800 to 900 feet in thickness. It contains about thirty seams of coal above a foot thick, and many more of less size. They occur irregularly, some lying only a few inches apart, others from eighty to ninety feet, the intervening space being occupied by sandstone or shale. Now as each coal-seam, with its associated under-clay, appears to mark a former land surface, it will follow that there must be as many old land surfaces in this series of strata as there are such coal-seams, and that for every intervening mass of sandstone or shale, the area of vegetation must have been submerged. This conclusion would have been violently resisted by the supporters of the "drift" theory. They would have roundly asserted that such an unsteady surface was a mere supposition to suit a hypothesis, unsupported by fact, and contrary to the analogy of existing nature; and they would not perhaps have hesitated to maintain, that such an oscillating land could be little fitted to nourish so rich and luxuriant a vegetation as that of the Carboniferous period. But it will not be difficult to show that our conclusion, so far from being contrary to analogy, is amply borne out by the processes of existing nature, and that its opponents, and even its original asserters, failed to perceive that what it demands is not a rapidly oscillating crust, but one as steady and uniform as that of many of the least disturbed countries at the present day; and that we do not require to call in the aid of a special elevation and submergence for every coal-seam, but that for the most part the hypothesis of a steady sinking of the area of a coal-field, interrupted perhaps by occasional elevatory movements, along with an active and constant deposition of sediment by the varying currents of a large river, is sufficient, if not thoroughly to explain, at least to throw great light upon the origin of those enormous masses of strata composing our present coal-basins. The oft-recurring variations in