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Show 17G OLDER PLIOCENE PERIOD· [Ch. XIII. '\IVhen we inquire into the causes of such a disposition of the No.3:>. materials of each bed or group of lay- ~,~~'· ·· ers, we may, in the first place, remark, ... r/ ~· that however numerous may be the suc-cessive layers a, b, c, the layer a must have been deposited before b, b before c, and so of the rest. '\Ve must suppose that each thin seam was thrown down on a slope, and. that it conformed itself to the side of the steep bank, just as we see the materials of a talus arrange themselves at the foot of a cliff when they have been cast down successively from above. If the transverse layers are cut off by a nearly horizontal line, as in many of the above sections, it may arise from the denuding action of a wave which has carried away the upper portion of a submarine bank and truncated the layers of which it was composed. But I do not conceive this hypothesis to be necessary; for if a bank have a steep side, it may grow by the successive apposition of thin strata thrown down upon its slanting side, and the removal of matter from the top may proceed simultaneously with its lateral extension. The same current may borrow from the top what it gives to the sides, a mode of formation which I had lately an opportunity of observing on the rippled surface of the hills of blown sand near Calais. 'rhe un. dulating ridges and intervening furrows on the dunes of blown sand resembled exactly in form those caused by the waves on a sea-beach, and were always at right angles to the direction of the wind which had produced them. Each ridge had one No. 36, b d e side slightly inclined and the other steep, the lee side being always steep, as b c, de, the windward side a gentle slope, as a b, c d. When a gust of wind blew with sufficient force to drive along a cloud of sand, all the ridges were seen to be in motion at once, each encroaching on the furrow before it, and, in the course of a few minutes, filling the place which the fur- Ch. XIII.] DERANGEMENT T~ THE CRAG STRA'l'A. 177 rows had occupied. Many grains of sand were drifted alonO' the slopes a b, and c d, which, when they fell over the scarp~ b c, :nd d e, ';ere under ~helter from the wind, so that they remamed statiOnary, resting, according to their shape and momentum, on different parts of the descent · In tl11' s manner each ridge was distinctly seen to move slowly on as often as the force of the wind augmented. We think that we shall not straifl analogy too far if we suppose the same laws to govern t~1e s~baqueous an~ subaerial phenomena; and if so, we may Imagme a submarme bank to be nothing more than ~nc of the ridges of ripple on a larger scale, which may increase m the manner before suggested, by successive additions to the steep scarps. The set of tides an~ ~urre.nts, in opposite directions, may account for sudden van:twns m the direction of the dip of the layers, as represented m the wood-cut, No. 33, while the general prevalence of a southerly inclination in the Crag of Suffolk may indicate that the matter was brought by a current from the north. We may refer to a drawing given in the first volume*, to show the analogy of the arrangement of the submarine strat just considered, to that exhibited by deposits formed in th~ chann.el~ of rivers where a considerable transportation of sediment Ism progress. Derangement in the Crag sfrata.-In the above exampl we have e~plained the want of parallelism or horizontality ;: the subordmate layers of different strata, by reference to the mode of the~r original deposition ; but there are signs of disturbance winch can only be accounted for by subsequent movements. 'rhe same blue and brown clay, or loam, which is often p1er fectly hor.i zontal, and as regularly bedded as any o f our o der formattons, is, in other places, curved and even folded back up on I· t se If, l·l l t 11 e manner represented in the annexed diagmms. * Chap. xiv., Ding. No.6. Vor., III. N |