_On Fossil Rain-marks of the Carboniferous Period in North America._--Having alluded to the spots left by rain on the surface of carboniferous strata in the Alleghanies, on which quadrupedal foot-prints are seen, I may mention that similar rain-prints are conspicuous in the coal measures of Cape Breton, in Nova Scotia, in which Mr. Richard Brown has described Stigmariae and erect trunks of trees, and where there are proofs, as stated at p. 324., of many fossil forests ranged one above the other. In such a region, if anywhere, might we expect to detect evidence of the fall of rain on a sea-beach, so repeatedly must the conditions of the same area have oscillated between land and sea. The intercalation of deposits, containing shells of marine or brackish water, indicate the constant proximity of a body of salt water when the clays which supported the upright trees were formed. In the course of 1851, Mr. Brown had the kindness to send me some greenish slates from Sydney, Cape Breton, on which are imprinted very delicate impressions of rain-drops, with several worm-tracks (_a_, _b_, fig. 526.), such as usually accompany rain-marks on the recent mud of the Bay of Fundy, and other modern beaches.[xii-B] [Illustration: Fig. 526. Carboniferous rain-prints with worm-tracks (_a_, _b_) on green shale, from Cape Breton, Nova Scotia.] [Illustration: Fig. 527. Casts of rain-prints on a portion of the same slab, No. 526. seen on the under side of an incumbent layer of arenaceous shale. The arrow represents the direction of the shower.] [Illustration: Fig. 528. Casts of carboniferous rain-prints and shrinkage-cracks, (_a_) on the under side of a layer of sandstone, Cape Breton, Nova Scotia.] The casts of rain-prints, in figs. 527. and 528., project from the under side of two layers, occurring at different levels, the one a sandy shale, resting on the green shale (fig. 526.), the other a sandstone presenting a similar warty or blistered surface, on which are also observable some small ridges as at _a_, which stand out in relief, and afford evidence of cracks formed by the shrinkage of subjacent clay, on which rain had fallen. Many of the associated sandstones are described by Mr. Brown as ripple-marked. The great humidity of the climate of the coal period had been previously inferred from the nature of its vegetation and the continuity of its forests for hundreds of miles; but it is satisfactory to have at length obtained such positive proofs of showers of rain, the drops of which resembled in their average size those which now fall from the clouds. From such data we may presume that the atmosphere of the carboniferous period corresponded in density with that now investing the globe, and that different currents of air varied then as now, in temperature, so as to give rise, by their mixture, to the condensation of aqueous vapour.