Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/307

Rh sTP.ATiF1cA'r1oN.] workmen are constantly dischargingwaggon-loads of rubbish. I Hence the embankment, if cut open longitudinally, would present a “ false-bedded ” structure, for it would be found to consist of many irregular layers inclined at a high angle in the direction in which the formation of the mound had alvanced. In the accompanying ﬁgure (ﬁg. 10) the water L)" ,— 5:‘?- l-‘ic. lO.—Scction illustrating the production of false-bcdiliiig. moving in the direction of the arrow may drop sand at b, which will corresponrl in lamination with the general stratification of the locality; but when the current reaches the steep front of one of the advancing sand sheets_ it will allow the sand to roll down the slope, and may continue to bring fresh supplies of sediment until the slope is gradually ell’-.iced. Now and then, however, instead of laying down se;liincnt-, a current of greater strength than usual may appear and sweep away portions of the sediment already deposited. Ir2'u_-/ulun'(ies qf ]}eilrh'7zg clue to I aequal ities of Deposition or 1y‘ ]'.'ro.sz.'0u,.-—A sharp ridge of sand or gravel may be laid down under water by current-actioii of some strength. Should the motion of the water diminish, ﬁner sediment intiy be brought to the place and be deposited around and above the ridge. In such a case the stratiﬁcation of the later accumulation will end off abruptly against the flanks of the older ridge, which will appear to rise up through the overlying bed. In ﬁg. 11, for example, the lower bed ,:J-<:::. ' . - — a Fic. 11.—.Iound of clay with ironstone balls ((1), covered by beds of coal ((15). seems to have been locally heaped up into the shape of a moimd or ridge before the coal was accumulated over it. Appearances of this kind are not uncommon in some coal- ﬁclds, where they are known to the miners as “rolls,” “ swells,” or “ horses’ backs.” A structure exactly the reverse of the preceding occurs where a stratum has been scooped out before the deposition of the layers which cover it. This has often been observed in mining for c0a[_ Channels have been cut out of a coal-seam, or rzcither out of the bed of vegetation which ultimately became coal. and these channels, ramifyiiig and winding sometimes like those of streamlets on ﬂat ground, have been ﬁlled up with sandy I-‘in. l2.—Sccti0n_ of New Red Sainrlstnnc. road-cutting near 'olvcrli:iniptoii. 1, H-rd and white clay oi'_marl; 2, Brown sandstone with il'l‘e_Elll:ll' ratchus of inzirl; 3, Red marl, partially eroded before the depositioii of 4. Brown sand- stone eroded before the formation of 5, Calcarcous sandstone or cornstoue. or muddy sediment. In fig. 12 a section is given of a remarkable series of such erosions, where beds of clay and sandstone have been extensively denuded in the intervals GEOLOGY 29:} between the deposit of the successive beds. In these and similar cases it is evident that the erosion took place coii- temporaneously with the accumulation of the deposits as {L whole. We cannot tell, of course, how long an interval elapsed between the formation of a given stratum and that of the next stratum which lies upon its eroded surface, nor how much depth of rock may have been removed in the erosion. When, however, as in the instances with which we are dealing, the structure occurs among conformable strata, evidently united as one lithologically continuous series of deposits, we may reasonably infer that the missing por- tions are of small moment and that the erosion was merely due to the irregular and more violent action of the very currents by which the sediment of the successive strata was supplied. The case is very different when the eroded strata are inclined at a different angle to those above them, and are strongly marked off by lithological distinctions. In some of the coal-mines in central Scotland, for instance, deep channels have been met with entirely ﬁlled with sand, gravel, or clay belonging to the general superﬁcial drift of the country. These channels have evidently been water- courses worn out of the coal-measure strata at a coinpara— tively recent geological period, and subsequently buried under the glacial accumulations. There is a complete dis- cordance between them and the Palaeozoic strata below, pointing to the existence of a vast interval of time. I3z'pple-mm'l'.—Tlie surface of many beds of sandstone is marked with lines of wavy ridge and hollow, such as may be seen on any shore from which the tide has retired. This kind of surface is known as “ ripple-niark.” It may be formed on dry blown sand by the action merely of the wind, and it is of everyday occurrence under shallow water, not merely on sea—shores, but on the floors of lakes and of river-pools. The water, gently agitated by the wind in a given direction, throws the surface of the underlying sediment into ripples which tend to rim at right angles to the course of movement. But as the wind veers from point to point, producing corresponding changes in the direction of the watcr-currents, the ripples on the bottom are not strictly parallel, but often coalesce, intersect, and undulatc in their course. Their general direction, however, suﬂices to indicate the quarter whence the chief movement of the water has come. No satisfactory inference can be drawn froiu the existence of a rippled surface as to the depth of -water in which the sediment was accumulated. As a rule it is in water of only a few feet or yards in depth that ripple-mark is formed. But it may be produced at any depth to which the agitation caused by wind on the upper waters may extend. On an ordinary beach each tide usually eﬂ'aces the ripple- marks made by its predecessor, and leaves a. new series to be obliterated by the next tide. But where the markings are formed in water which is always receiving fresh accumula- tions of sediment, a rippled surface may be gently over- spread by the descent of a layer of sediment upon it and may thus be preserved. Another series of ripples may then be made in the overlying layers, which in turn may be buried and preserved under a renewed deposit of sand. In this way a considerable thickness of such ripple-marked strata may be accumulated, as has frequently taken place among geological formations of all ages. An examination of any sandy beach from which the sea has recently retired brings before us many modiﬁcations of the perfect ripple-mark. The ridges may be seen to grow more and more notched and irregular, until at last the beach seems to be dotted over with little, ﬂat, dome—shaped mounds, or as if the ridges of the ripple-mark had been fiirrowcd across. These modiﬁcations are doubtless due to the partial effacement of the ridges by subsequent action