remember that, as the mud dries and contracts it splits up into a network of cracks; and that, on its hardened surface, it retains impressions of the feet of birds or of insects that may have walked over it while still soft. The geological history recorded at such places cannot be mistaken; first, the rainy period, with the rush of muddy water down the slopes and the formation of pools in which the mud is allowed to settle; then the season of warm weather when the pools gradually dry up and birds seek their edges to drink. If by any means a layer of sediment could be

laid down upon one of these desiccated basins so gently as not to efface its peculiar markings, the cracked surface of mud, with its footprints, would contain a perfectly intelligible record of the changes which it had witnessed (Fig. 83).

Now surfaces of this kind abound among the sedimentary rocks of the earth's crust. They are found upon strata which, from the presence of marine organic remains in them, were certainly deposited under the sea. But these strata cannot have accumulated in deep water; they must have been formed along flat shores, where the sheets of sand and mud were liable from time to time to be laid bare to the sun and wind, where animals of various kinds left their footmarks or trails on the still soft

sediment, where the evaporation and desiccation were so rapid as to cause the exposed mud to harden on the surface and to crack up into irregular polygonal cakes, and where the next succeeding layers of sediment were deposited so gently as to cover up and preserve the sun-cracked surfaces.

One further piece of evidence to indicate land-surfaces, or, at least, shore-surfaces, in a series of aqueous sedimentary strata, is that furnished by Rain-prints. A brief shower of rain leaves upon a smooth surface of fine sand or mud a series of small pits,

each of which is the imprint of a descending raindrop (Fig. 84). Where this takes place along the edge of a muddy pool which is rapidly being dried up, the prints of the drops may remain quite distinct on the hardened surface of mud. And here, again, we can suppose that if another layer of mud were gently deposited above this surface the rain-prints would be sealed up and preserved. We might even be able to tell from what quarter the wind blew that brought the rain-cloud. If, for example, the rain-prints were ridged up on one side in one general direction this would show that the shower fell aslant and with some force, and that the side on which the mud round the imprints was forced up was that towards which the rain was driven. Such indications of ancient weather may here and there be detected among stratified rocks.

Concretions. Another original characteristic of many sedimentary rocks is a concretionary structure, particularly observable

N

in clays, limestones, and ironstones. In many cases, the concretions have gathered round some fragment of a plant or an animal. Clay-ironstone and impure limestone have been aggregated into spherical or elliptical forms (septaria), which are of frequent occurrence in clay or shale (Figs. 61, 65). Flint has also gathered round some organic nucleus, which it has often entirely replaced. But many concretions may be found where no organic fragment as a starting-point can be detected. Some of the most curious are the so-called Fairy-stones (Fig. 64), found in alluvial clays, with so many imitative shapes, which have been

popularly supposed to be works of human or even preternatural construction. They have probably been produced by the irregular cementing of clay, owing to the spread of carbonate of lime through it, carried down by permeating water. Some of the most extraordinary concretionary masses are to be seen in certain magnesian limestones, which appear to be built up of petrified lumps of coral, bunches of grapes, cannon-balls, and other objects (Fig. 75). In reality, all these diversified figures are due to the irregularly varied way in which a concretionary structure has been developed in the limestone.

Association and Alternation of Strata.-Certain kinds of sedimentary rocks are apt to occur together to the exclusion of others. This association depends on the circumstances of

deposition. Ironstone concretions, for example, are much more frequent among clays or shales than in any other strata, because it was during the deposit of fine mud with abundant decomposing organic matter that the most favourable conditions were supplied for the precipitation of carbonate of iron. Clays and limestones frequently alternate, as also do sandstones and conglomerates, because the circumstances of deposition were somewhat alike (see Fig. 80). But we need not expect to encounter a bed of coarse conglomerate in a group of fine clays, for the current that was strong enough to sweep along the stones of the conglomerate was too powerful to allow the fine silt to lie undisturbed. For a similar reason, we should be surprised to meet with a layer of well-stratified shale in a mass of conglomerate. The agitated water in which these coarse materials were heaped up would have swept away any fine sediment and prevented it from being deposited. In all cases, the manner in which the different kinds of sediment are associated with each other leads us back directly to the original conditions of deposit, and is only intelligible in proportion as these conditions are clearly realised.

Relative Areas of Stratified Rocks. Moreover, some kinds of sedimentary material must obviously spread over wider areas than others. The coarse gravel and shingle of the present beach do not extend far seawards; they are confined to the margin of the land. Sand covers the sea-floor over a wider area; and beyond the limits of the sand, in the deeper and stiller water, mud is allowed to accumulate. Roughly speaking, therefore, the area of the distribution of sediment is in inverse proportion to the coarseness of the materials. The same law has regulated the accumulation of detritus from early geological time. Coarse conglomerates, which represent ancient shingles and gravels, thicken and thin out rapidly, and do not usually cover a large area, though they may sometimes be traced for long distances in the direction probably of the original coast or line of heaping up of the shingle. They pass laterally and vertically into grits and sandstones which have a much wider distribution, and these again shade off into clays and shales that range also over large

areas.

Chronological Value of Strata.-No clue has yet been found to determine the length of time required for the accumulation of a stratum or group of strata; but some indications are afforded of relative lapse of time. Here and there, for instance, vertical trunks of trees are met with standing in their positions of growth,

but imbedded in solid sandstone (Fig 85). These stems, sometimes 20 feet or more in height, prove that a mass of sand of that depth must have been accumulated around them before they had time to decay. We know little about the durability of the submerged trees; but they probably could not have lasted long unless covered up by sediment; so that the mass of strata in which they are enclosed may be supposed to have been accumulated within a few years. The nature of the material composing

sedimentary rocks may likewise furnish indications of relative rate of deposition. Thus finely laminated clays were evidently deposited with extreme slowness. Beds of limestone, composed of the crowded remains of successive generations of marine creatures, must also have required prolonged periods of time for their growth. On the other hand, thick beds of sandstone presenting great uniformity of characters may not improbably have been laid down with comparative rapidity.

No reliable inference can be drawn from the mere thicknesses of strata as to the lapse of time which they represent. A mass