of coal, and that it was merely a case of diagonal bedding. Similarly, near Corsham, in Wiltshire, a group of Oolitic limestones may be seen resting obliquely on other beds of shelly limestone, which are nearly horizontal.

(2.) Curvilinear or Irregular Bedding. This structure is produced under similar circumstances to that just described, but should be distinguished from it, inasmuch as it is always a proof of frequent change in the velocity and direction of the currents by whose action the deposits were accumulated. Diagonal bedding is produced by the action of a steady current flowing continuously in one direction. Curvilinear bedding is caused by the meeting of currents which vary in force and direction from time to time. The term False-bedding has been applied to both structures, but is a misleading term, for both are truly bedded, though the inclination of the beds may be deceptive.

Sir Charles Lyell has described the appearance of a cutting through a sandbank formed near a junction of the rivers Arve and Rhone, where the conflicting currents had produced a series of inclined beds with curvilinear surface; above these were more or less horizontal layers, surmounted by irregular alternations of sand and gravel in undulating curvilinear layers. Irregular bedding of this nature is the result of constantly shifting and conflicting currents, and is seldom exhibited by other deposits than those formed in shallow water, viz., sand and gravel, or sandstone and conglomerate, though it is not unfrequently developed in certain limestones which chiefly consist of the rolled and drifted débris of shells and corals, as, for instance, the great Oolite limestone of Bath and Minchinhampton, and the rock known as the Lincolnshire limestone.

Fig. 93 represents the arrangement of the beds in the face of a quarry at Minchinhampton, as sketched by Mr. Jukes. Part only of the quarry face is shown in fig. 93, the top line being a marked plane of bedding; each bed is obliquely laminated, but the surfaces of the beds are all more or less curvilinear; it is evident also that none of these beds are complete, but are portions of so many banks 1 See figure in Lyell's "Principles of Geology," ch. xix. fig. 44. See "Popular Physical Geology," Jukes, pl. 8.

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of coral sand formed by the agency of local currents which frequently changed their direction, and constantly removed and rearranged the materials they acted upon. The contrast and variety is often heightened by differences in the size or colour of the particles composing the different beds. Current Mark or Ripple Drift.-Another effect of current-action is to be found in ripple-marked surfaces. The rippled surface is similar to that so often seen on the sands of a sea-shore, where it is produced, not by the rippling of the waves as they break on the strand, but by the action of the tidal current which has passed over it,

Fig. 93. Curvilinear Bedding in Great Oolite, Minchinhampton.

and because any current passing over a yielding surface develops an undulating movement. Wind moving over the surface of water causes a ripple on that surface. Water moving over the surface of fine sand causes a similar ripple upon it.

The formation of ripples on the sandy bottom of a brook has been described on p. 130, and any current running steadily in one direction produces similar ripples, the particles on the top of each ridge being continually pushed over its edge, while others are pushed up the slope behind. In this way a succession of little inclined laminæ or stratulæ are formed, arranged in a series of parallel bands or layers which slope slightly towards the direction from which the material is brought, and consequently in an

opposite direction to the inclination of the small stratulæ of which they are composed (see fig. 94). Each of these bands ends in a surface ridge or ripple.

Mr. Sorby states that in fine-grained sandstones which exhibit ripple-drift, the small stratulæ of sand are often separated by thin layers of argillaceous matter, the different bands being also separated by a similar deposit. He also finds a similar structure in beds of mica-schist, where the quartzose folia correspond to the sandy layers and the micaceous to the argillaceous layers, and showing that these schists were originally beds of shaley or flaggy sand

stone.

An oscillating movement will also produce a ripplemarked surface, though not a regular succession of rippledrift layers. This can be tested by oscillating a basin of water into which sand has been thrown. Waves produced by the wind produce an oscillating disturbance of the

m, Ripple marks. b, Ripple-drift layers or laminæ.

material on the sea-floor, and as the influence of large waves extends to a considerable depth, ripple-marks may be produced in any depth at which this influence is appreciable. Tidal currents also disturb the bottom in fairly deep water under certain conditions (see ante, p. 182). Ripple-mark has in fact been observed beneath 50 to 60 feet of water.

Mr. Jukes observes that in places where the current is troubled, "a modification of these rippled surfaces is sometimes produced, the bed being irregularly mamillated on its surface, which is pretty equally, though irregularly, divided into small hollows and protuberances of a few inches diameter. This surface structure may be seen in process of production now on shores where spaces of sand are enclosed by rocks, so that as the tide falls it is made to run in different directions among the rock channels; but it would probably be caused at any depth at which a

current could be similarly troubled and confused. This might be called 'Dimpled current-mark.'"

It must be borne in mind that the rippled surfaces which are formed between tide-marks can seldom be preserved, because the incoming tide generally obliterates those previously formed. It is probable, therefore, that the ripplemarks preserved on ancient rock-surfaces have in most cases been formed beneath shallow water at a greater or less distance from low-tide level. A ripple-mark produced by the tidal current may be covered with the sediment which is deposited during the slack of the tide, and the new surface thus formed may be similarly rippled by the gentle movement of the return current, so that every successive layer of lamina will be ripple-marked. In finegrained sandstones or sandy shales it is not unusual to find a succession of ripple-marked surfaces, one under the other, at distances of a few inches apart; and the direction of the ripples somewhat varies considerably on the different surfaces.

Mr. Sorby has shown that inferences may be drawn from the examination of these current-marks as to the velocity and direction of the currents which caused them, and that we may thus reason back to some conclusions regarding the physical geography of the district at the time they were formed.

Although, as above stated, ripple-marked beds are not usually such as were formed between tide-levels, yet surfaces are occasionally found which exhibit not only ripplemarks, but also sun-cracks, rain-prints, worm-tracks, and the footmarks of the various creatures which travelled over the moist, exposed surface of the bed. Such surfaces have doubtless been formed on flat shores, where large tracts were exposed between the lines reached by neap and spring tides, so that the surface beyond high water of neap tides, being exposed to the action of the sun, became cracked and hardened, and the next spring tide only spread over it another layer of sand or mud. Among lacustrine deposits they have doubtless been formed during dry seasons, when the lake waters shrank and large surfaces of the lake-floor were temporarily exposed.

IN

CHAPTER VI.

CONSOLIDATION OF ROCKS.

N the first part of this volume (Chapters XI. to XVI.) the manner in which deposits are now being formed in rivers, lakes, and seas has been explained; these being for the most part loose and unconsolidated sands, gravels, clays, and muds. In Part II., Chapter IV., the principal sedimentary rocks which are found in the earth's crust have been described, many of these being very hard and compact materials. It is natural that at this stage the student should ask how it is that deposits which were originally soft and incoherent, like most of those now being formed, have become so hard and consolidated as to be termed in ordinary language rock or stone. In the present chapter we shall endeavour to give some answer to this question. The processes by which stratified rocks are consolidated may be mentioned under the following heads :

1. Desiccation.

2. Pressure.

3. Infiltration.

4. Chemical change.

5. Heat.

:

1. Desiccation.-All stratified rocks which have been formed at the bottom of lakes and seas must of course be saturated with water for a long time after their deposition, as long, in fact, as the lake or sea remains above them. When at length earth-movements take place, and the subaqueous deposits are converted into land, they are partially drained of the included water, and their materials consequently settle down into a smaller space.