instances, the animal life gradually became adjusted to the sea, the greater part, if not the whole, of the previous deposits in the simply overtopped lake might be preserved and be covered by the brackish water, and finally by the marine accumulations. The unequally-tilted lake banks might permit a part of the older deposits to be so exposed to breaker action that they were partially removed, the component mineral matter and its organic contents partially also rearranged with the new accumulations. If, during a re-establishment of part of North America beneath the sea, it so occurred that Lakes Erie and Ontario were depressed more rapidly than Lakes Huron, Michigan, and Superior, the sea finally overspreading the whole, the relative positions of the lakes to the direction of the greatest depression would much influence the results. Lakes Erie and Ontario would present their breadths to the movement, while Lakes Michigan and Huron would be acted upon in their lines of length, Lake Superior presenting a more complicated form. Under such a movement, the entrance of the sea would necessarily depend upon the varied surface and levels for the time opposed to it; but it may readily happen that while Lakes Ontario and Erie were beneath the sea, and Lake Huron brackish water, Lake Superior might continue as fresh water, the contemporaneous deposits in each containing the remains of animals capable of living in the various kinds of water respectively, such of the original lacustrine creatures remaining in the brackish water as could adjust themselves to it, mingled with those marine animals which could support life under the same conditions, the terrestrial vegetation drifted into all the deposits being of the same general kind.

CHAPTER XXVII.

MODE OF ACCUMULATION OF DETRITAL AND FOSSILIFEROUS ROCKS CONTINUED. -EVIDENCE AFFORDED BY THE COAL-MEASURES.-STEMS OF PLANTS IN THEIR POSITION OF GROWTH.-FILLING UP OF HOLLOW VERTICAL STEMS, AND MIXTURE OF PROSTRATE PLANTS WITH THEM.-GROWTH OF PLANTS IN SUCCESSIVE PLANES.-THICKNESS OF SOUTH WALES COAL MEASURES.FALSE BEDDING IN COAL MEASURE SANDSTONES.-SURFACE OF COALMEASURE SANDSTONES.-DRIFTS OF MATTED PLANTS IN COAL MEASURES. -EXTENT OF COAL BEDS.-PARTIAL REMOVAL OF COAL BEDS.-CHANNELS ERODED IN COAL BEDS, FOREST OF DEAN.-LAPSE OF TIME DURING DEPOSIT OF COAL MEASURES.-PEBBLES OF COAL IN COAL ACCUMULATIONS.MARINE REMAINS IN PART OF THE COAL MEASURES.-GRADUAL SUBSIDENCE OF DELTA LANDS.-FOSSIL TREES AND ANCIENT SOILS, ISLE OF PORTLAND.-WEALDEN DEPOSITS, SOUTH-EASTERN ENGLAND.-RAISED SEA-BOTTOM ROUND BRITISH ISLANDS.-OVERLAP OF CRETACEOUS ROCKS IN ENGLAND.

WHILE the remains of drifted terrestrial plants, large or small, may not give very exact information as to the area occupied by dry land, whence they have been derived, since they could have floated from considerable distances (p. 125), according to the currents of particular geological times, where these remains occur either in their places of growth, or so that we may rightly conclude that they have not been removed far from them, they become important. Those deposits of vegetable matter interstratified with shales, sandstones, and conglomerates, which occur in a nortic portion of the geological series America, and to which the t from abundantly furnishing

to the progress of civiliz

The Gulf Stream, as be water capable of transporti regions of the north across

ferring the existence of land in particular portions of the northern hemisphere at that time. When carefully examined, a large proportion of the coal beds have been found, in the British Islands (and the evidence would also appear to justify similar conclusions in many other countries), resting upon others immediately beneath, in which the roots of particular plants are found to extend in a manner showing that these are actually in their places of growth, as respects the beds of mineral matter containing them. These roots were at one time considered as separate plants (Stigmaria), but now, from the researches of Mr. Binney and other geologists, it seems established that they belong to other plants (Sigillaria, if not also to other genera). With this advance of knowledge, we find that great sheets of vegetable matter were based upon a mud or silt, in which the amount of sand varied considerably in different situations, even in the prolongation of the same bed, and that into this mud or silt the roots of at least some of the plants of the time and locality spread as in ground for which they were suited.

Upon further investigation, it has been found that roots of this character are to be seen attached to stems of plants still vertical, or nearly so, to the beds of shale or sandstone (formerly mud, silt, or sand), in which they are enclosed. Though the attachment of such roots may be rarely seen, the examples of vertical stems of plants, apparently in their places of growth, are sufficiently common, so much so that if certain parts of the coal measures of the British Islands could have the detrital matter removed, various and extensive areas would be found covered by the stumps of plants in such positions. These stumps are so numerous in the ordinary detrital deposits reposing on some coal beds, that they become dangerous in the collieries, (unless great care be taken in the works,) from being merely sustained aloft by the coaly matter representing the former outer portion of the plants, so that when this is insufficient to retain them, they fall on the heads of the miners. The following sketch (fig. 182), at Cwm Llech, towards the head of the Swansea valley, Glamorganshire,* may serve to illustrate the manner in which these plants may sometimes be exhibited in quarries or natural cliffs, rising amid the beds which have enveloped them in their places of growth. The largest of the two stems was 5 feet in circumference. They merely formed a part of a surface more or less covered by stems of

Made by Mr. Logan, by whom and the author the locality was carefully examined. The stems were subsequently removed to the Royal Institution of South Wales, at Swansea, where they now are.

this kind, as others were to be seen in similar positions in the same bed of rock higher up in the same valley. Upon uncovering a

shale beneath the sandstone, in which these plants (Sigillaria) stood, an abundance of fern leaves, and fragments of other plants, commonly seen in these deposits, were found distributed around in the same manner as leaves and other parts of plants may be dispersed around stems of trees in muddy places at the present day.

It sometimes happens that the vertical stems of the plants rise through different kinds of beds, the component parts of which accumulated around them, while the vegetable matter still held together. The following (fig. 183) is an example of this kind, as it was exhibited at the Killingworth Colliery, Newcastle district. In this section a represents the high main coal of the district, b argillo-bituminous shale (formerly carbonaceous mud), e blue shale (mud or clay), d compact sandstone (sand), e alternating shales and standstones (beds of mud and sand), h white sandstone (clean sand), i micaceous sandstone (sand with mica), and k shale (mud or clay). In such cases various changes were effected in the kind of mineral matter transported to, and deposited amid, the vegetation there standing. Though we do not know the extent to which such plants may have been covered up before they died, an attentive study of the mode in which the mud, silt, or sand has been accumulated round the stems often shows the observer that the water bearing or moving the detritus was very shallow.

Around the stems at Cwm Llech (fig.182), the lamina of the sandstone were so arranged as forcibly to suggest that they represented

the washing up of sands around the plants in shallow water agitated by slight waves. Such an arrangement may frequently be seen, as also occasionally, when the stems are carefully uncovered, an adjustment of the lamina of the original sand or silt, in a manner pointing to the passage of a slight current of water by them. When this can be found, the direction whence the current came may be inferred by the position of the lamina marking the place of the eddy, behind the stems.

From the manner in which these vertical stems are so frequently terminated upwards, it would appear that while, for a time, their lower portions continued to resist the pressure both of the water in which they were immersed, and the gradually-accumulating detritus borne or drifted by it, their tops became decayed, and were removed, so that finally sheets of detritus uninterruptedly spread over the localities where such plants may have grown. We seem, indeed, to have evidence, in the manner in which so many of these stems have been filled with mud, silt, sand, and the remains of other plants, that before such sheets of continuous detritus were spread over their tops, they were hollow, like so many open and vertical tubes, in which, when overtopped by waters bearing detrital matter, and the leaves and fragments of plants, these were deposited in the same way that sediment and the remains of vegetation are accumulated in the hollows of upright and decayed or broken stems of bamboos, and other plants on the side of rivers,