36. _Mud-cracks and Rain-prints._--The surfaces of some beds occasionally exhibit markings closely resembling those seen upon a flat sandy beach after the retreat of the tide--hence they are called _ripple-marks_ or _current-marks_. They are, of course, due to the gentle current action which pushes along the grains of sand, and hence, such marks may be formed wherever a current sweeps over the bottom of the sea with energy just sufficient for the purpose. But since the necessary conditions for the formation of _ripple-mark_ occur most abundantly in shallow water, its frequent appearance in a series of strata may often be taken as evidence, so far, for the shallow-water origin of the beds. Besides ripple-marks we may also detect occasionally on the surfaces of certain strata _mud-cracks_ and _rain-prints_. These occur most commonly in fine-grained beds, as in flagstones, argillaceous sandstones, shales, &c. The _mud-cracks_ resemble those upon a mud-flat which are caused by the desiccation and consequent shrinkage of the mud when exposed to the sun. The old cracks have been subsequently filled up again by a deposition of mud or sand, usually of harder consistency than the rock traversed by the cracks. Hence, when the bed that overlies the mud-cracks is removed, we find a cast of these projecting from its under surface, or frequently the cast remains in its mould, and forms a series of curious ridges ramifying over the whole surface of the old mud-flat. _Rain-prints_ are the small pits caused by the impact of large drops. They are usually deeper at one side than the other, from which we can infer the direction of the wind at the time the rain-drops fell. Like the mud-cracks, they are most commonly met with in fine-grained beds, and have been preserved in a similar manner. Some geologists have also been able to detect _wave-marks_, 'faint outlinings of curved form on a sandstone layer, like the outline left by a wave along the limit where it dies out upon a beach.' 37. _Succession of Strata._--The succession of strata is often very diversified. Thus, we may observe in one and the same section numberless alternating beds of sandstone and shale from an inch or so up to several feet each in thickness, with seams of coal, fireclay, ironstone, and limestone interstratified among them. In other cases, again, the succession is simpler, and some deep quarries shew only one bed, as is the case with certain limestones, fine-grained sandstones (liver-rock), and many volcanic rocks. Some limestones, indeed, shew small trace of bedding throughout a vertical thickness of hundreds of feet.