nut has only been overcome by the application of the principle of changes of level in the surface of the earth, and in this we shall find a sure explanation of the phenomena of the coal-measures.

Great changes of the level of the land are undoubtedly taking place even now on the earth's surface, and in assuming that similar changes took place in carboniferous times, we shall not be assuming the former existence of an agent with which we are now unfamiliar. And when we consider the thicknesses of sandstone and shale which intervene beneath the coal-seams, we can realise to a certain extent the vast lapses of years which must have taken place between the existence of each forest; so that although now an individual passing up a coal-mine shaft may rapidly pass through the remains of one forest after another, the rise of the strata above each forest-bed then was tremendously slow, and the period between the growth of each forest must represent the passing away of countless ages. Perhaps it would not be too much to say that the strata between some of the coal-seams would represent a period not less than that between the formation of the few tertiary coals with which we are acquainted, and a time which is still to us in the far-away future.

The actual seams of coal themselves will not yield much information, from which it will be possible to judge of the contour of the landmasses at this ancient period. Of one thing we are sure, namely, that at the time each seam was formed, the spot where it accumulated was dry land. If, therefore, the seams which appear one above the other coincide fairly well as to their superficial extent, we can conclude that each time the land was raised above the sea and the forest again grew, the contour of the land was very similar. This conclusion will be very useful to go upon, since whatever decision may be come to as an explanation of one successive land-period and sea-period on the same spot, will be applicable to the eighteen or more periods necessary for the completion of some of the coal-fields.

We will therefore look at one of the sandstone masses which occur between the coal-seams, and learn what lessons these have to teach us. In considering the formation of strata of sand in the seas around our river-mouths, it was seen that, owing to the greater weight of the particles of the sand over those of clay, the former the more readily sank to the bottom, and formed banks not very far away from the land. It was seen, too, that each successive deposition of sand formed a wedge-shaped layer, with the point of the wedge pointing away from the source of origin of the sediment, and therefore of the current which conveyed the sediment. Therefore, if in the coal-measure sandstones the layers were found with their wedges all pointing in one direction, we should be able to judge that the currents were all from one direction, and that, therefore, they were formed by a single river. But this is just what we do not find, for instead of it the direction of the wedge-shaped strata varies in almost every layer, and the current-bedding has been brought about by currents travelling in every direction. Such diverse current-bedding could only result from the fact that the spot where the sand was laid down was subject to currents from every direction, and the inference is that it was well within the sphere of influence of numerous streams and rivers, which flowed from every direction. The only condition of things which would explain this is that the sandstone was originally formed in a closed sea or large lake, into which numerous rivers flowing from every direction poured their contents.

Now, in the sandstones, the remains of numerous plants have been found, but they do not present the perfect appearance that they do when found in the shales; in fact they appear to have suffered a certain amount of damage through having drifted some distance. This, together with the fact that sandstones are not formed far out at sea, justify the safe conclusion that the land could not have been far off. Wherever the current-bedding shows itself in this manner we may be sure we are examining a spot from which the land in every direction could not have been at a very great distance, and also that, since the heavy materials of which sandstone is composed could only be transported by being impelled along by currents at the bed of the sea, and that in deep water such currents could not exist, therefore we may safely decide that the sea into which the rivers fell was a comparatively shallow one.

Although the present coal-fields of England are divided from one another by patches of other beds, it is probable that some of them were formerly connected with others, and a very wide sheet of coal on each occasion was laid down. The question arises as to what was the extent of the inland sea or lake, and did it include the area covered by the coal basins of Scotland and Ireland, of France and Belgium? And if these, why not those of America and other parts? The deposition of the coal, according to the theory here advanced, may as well have been brought about in a series of large inland seas and lakes, as by one large comprehensive sea, and probably the former is the more satisfactory explanation of the two. But the astonishing part of it is that the changes in the level of the land must have been taking place simultaneously over these large areas, although, of course, while one quarter may have been depressed beneath the sea, another may have been raised above it.

In connection with the question of the contour of the land during the existence of the large lakes or inland seas, Professor Hull has prepared, in his series of maps illustrative of the Palaeo-Geography of the British Islands, a map showing on incontestible grounds the existence during the coal-ages of a great central barrier or ridge of high land stretching across from Anglesea, south of Flint, Staffordshire, and Shropshire coal-fields, to the eastern coast of Norfolk. He regards the British coal-measures as having been laid down in two, or at most three, areas of deposition—one south of this ridge, the remainder to the north of it. In regard to the extent of the former deposits of coal in Ireland, there is every probability that the sister island was just as favourably treated in this respect as Great Britain. Most unfortunately, Ireland has since suffered extreme denudation, notably from the great convulsions of nature at the close of the very period of their deposition, as well as in more recent times, resulting in the removal of nearly all the valuable upper carboniferous beds, and leaving only the few unimportant coal-beds to which reference has been made.

[Illustration: FIG. 23.—Cyathophyllum. Coral in encrinital limestone.]

We are unable to believe in the continuity of our coal-beds with those of America, for the great source of sediment in those times was a continent situated on the site of the Atlantic Ocean, and it is owing to this extensive continent that the forms of flora found in the coal-beds in each country bear so close a resemblance to one another, and also that the encrinital limestone which was formed in the purer depths of the ocean on the east, became mixed with silt, and formed masses of shaly impure limestone in the south-western parts of Ireland.

It must be noted that, although we may attribute to upheaval from beneath the fact that the bed of the sea became temporarily raised at each period into dry land, the deposits of sand or shale would at the same time be tending to shallow the bed, and this alone would assist the process of upheaval by bringing the land at least very near to the surface of the water.

Each upheaval, however, could have been but a temporary arrest of the great movement of crust subsidence which was going on throughout the coal period, so that, at its close, when the last coal forest grew upon the surface of the land, there had