Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/308

Rh 294 Such indications of shallow-water conditions may often be observed among old arenaceous deposits, as in the Cambrian and Silurian rocks. In like manner we may frequently detect, among these ancient formations, small isolated or connected linear ridges directed from some com- mon quarter, like the current-marks frequently to be found behind projecting fragments of shell, stones, or bits of sea-weed on a beach from which the tide has just retired. Sun-cracl-.9, 1i'ain-pittings, (f:c.—Proofs may not i11fre- quently be found that during deposition aqueous strata have been laid bare to air and sun. The nature and validity of this evidence will be best ascertained by observations made at the margin of the sea, or of any inland sheet of water, which from time to time leaves tracts of mud or ﬁne sand exposed to sun and rain. The way in which the muddy bottom of a dried—up pool cracks into polygonal cakes when exposed to the sun may be illustrated abundantly among geological formations of all ages. These desiccation-cracks, or sun-cracks, could not have been produced so long as the sediment lay under water. Their existence therefore, among any strata proves that the surface of rock on which they lie was exposed to the air and dried before the next layer of water-borne sediment was deposited upon it. With these markings are not infrequently associated ‘prints of rain-drops. The familiar effects of a heavy shower upon a surface of moist sand or mud may be witnessed among rocks even as old as parts of the Cambrian system. In some cases the rain-prints are found to be ridged up on‘ one and the same side in such a manner as to indicate that the rain-drops as they fell were driven along in a slanting direction. The prominent side of the markings therefore indicates the side towards which the wind blew. Numerous proofs of shallow shore-water, and likewise of exposure to the air, are supplied by markings left by ani- mals. Castings and trails of worms, tracks of mollusks and crustaceans, fin—marks of ﬁshes, footprints of birds, reptiles, and mammals, may all be preserved and give their evidence regarding the physical conditions under which sedimentary ‘formations were accumulated. It may frequently be noticed that such impressions are associated with ripple-marks, rain- prints, or sun-cracks; so that more than one kind of evi- dence may be gleaned from a locality to show that it was sometimes laid bare of Water. G'as-spm°ts.—Tl1e surfaces of some strata, usually of a dark colour and containing much organic matter, may be observed to be raised into little heaps of various indeﬁnite shapes, not, like the heaps associated with worm burrows, connected with pipes descending into the rock, nor composed of different material from the surrounding sandstone or shale. These may be conjectured to be due to the inter- mittent escape of gas from the decomposing organic matter in the original sand or mud, as we may sometimes witness in operation among the mud ﬂats of rivers and estuaries. On a small scale these protrusions of the upper surface of a deposit may be compared with the well-known mud-lumps at the mouths of the Mississippi, where the muddy bottom rises into mounds sometimes to a height of several yards above the Water, from the top of which great quantities of carburetted hydrogen gas make their escape, together with water and mud. C'oncretions.—)Iany sedimentary rocks are marked by the occurrence of concretions in them, either distinct in aspect and composition from the general mass of the rock, or forming really part of that mass, though separated from the rest by their being agglutinated into concretionary forms. Such concretions, where they differ in petrographical characters from the surrounding matter, are almost invari- ably of original or contemporaneous formation, that is, were formed at the same time as the strata among which they GEOLOGY of the water agitated by wind from a different quarter. [ lie. [iv. STRUCTURAL. Where, however, they appear to be merely com- pacted portions of the stratum, they may be regarded as generally due to some subsequent change effected upon the rock. Contemporancous concretions most commonly consist of carbonate of iron, carbonate of lime, or silica. Many clay- ironstone beds assume a nodular form, and this mineral occurs abundantly in the shape of separate nodules in shales and clay-rocks. The nodules have frequently formed round some organic body such as a fragment of plant, a shell, bone, or coprolite. That the carbonite of iron was slowly precipitated during the formation of the bed of shale in which its nodules lie may often be satisfactorily proved by the lines of deposit passing continuously through the nodules. In many cases the internal first-formed parts of a nodule have contracted more than the outerand more compact crust; and have cracked into open polygonal spaces which are commonly ﬁlled with calcite. Similar concretions of carbonate of lime occur in some clays and in connexion with limestones. Concretions of silica occur in limestone of many geological ages (see ante, p. 5239). The tlints of the English chalk are a familiar example, but similar siliceous concretions occur even in Lower Silurian limestones. The a. II’ f N raw z/(_.—."‘.-‘;. i_/'-.)‘_",,]__"’J/_, C. ' ~ n.//4 FIG. 13.—Skctch of limestone-hcds, with concretions of white chert, Middleton .oor. Derbyshirc. silica in these cases has not infrequently been deposited round organic bodies such as sponges, sea-urchins, and mollusca, which are completely enveloped in it and have even themselves been siliciﬁed. Iron-bisulphide (pyrite or marcasite) often assumes the form of concretions, more FIG. 14.—Sketch of part of a block of black chert in the limestone near Dublin. particularly among clay-rocks, and these, though prescntii g many eccentricities of shape, round like pistol-shot (.l' cannon—balls, kidney-shaped, botryoidal, &c., agree in usu- ally possessing an internal ﬁbrous radiated structure. Phosphate of lime is found as concretions in formations where the coprolites and bones of reptiles and other animals have been collected together. Concretions produced subsequently to the formation of the rock may be observed in some sandstones, which, when exposed to the weather, decompose into large round balls. Some shales exhibit this structure in a still more striking manner, inasmuch as the concretions consist of the general mass of the laminated shale, and the lines of stratiﬁcation pass through them and mark them out distinctly as super- induced upon the rock. -,c-me magnesian limestones are