490 COAL— RAIN-PKINTS. [Ch. XXIV. 



The casts of rain-prints in figs. 543 and 544, project from the under 

 side of two layers, occurring at different levels, the one a sandy shale, 

 resting on the green shale (fig. 542), 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 ripple-marked. 



The great humidity of the climate of the Coal period had been pre- 

 viously 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 vapor. 



The more closely the strata productive of coal have been studied, 

 the greater has become the force of the evidence in favor of their 

 having originated in the manner of modern deltas. They display a 

 vast thickness of stratified mud and fine sand without pebbles, and in 

 them are seen countless stems, leaves, and roots of terrestrial plants, 

 free for the most part from all intermixture of marine remains — cir- 

 cumstances which imply the persistency in the same region of a vast 

 body of fresh water. This water was also charged, like that of a great 

 river, with an inexhaustible supply of sediment, which seems to have 

 been transported over alluvial plains so far from the higher grounds 

 that all coarser particles and gravel were left behind. Such phenomena 

 imply the drainage and denudation of a continent or large island, hav- 

 ing within it one or more ranges of mountains. The partial intercala- 

 tion of brackish-water beds at certain points is equally consistent with 

 the theory of a delta, the lower parts of which are always exposed to 

 be overflowed by the sea, even where no oscillations of level are ex- 

 perienced. 



The purity of the coal itself, or the absence in it of earthy particles 

 and sand, throughout areas of vast extent, is a fact which appears very 

 difficult to explain when we attribute each coal-seam to a vegetation 

 growing in swamps. It has been asked how, during river inundations 

 capable of sweeping away the leaves of ferns and the stems and roots 

 of Sigillarice and other trees, could the waters fail to transport some 

 fine mud into the swamps ? One generation after another of tall trees 

 grew with their roots in mud, and their leaves and prostrate trunks 

 formed layers of vegetable matter, which was afterwards covered with 

 mud since turned to shale. Yet the coal itself, or altered vegetable 

 matter, remained all the while unsoiled by earthy particles. This 

 enigma, however perplexing at first sight, may, I think, be solved by 

 attending to what is now taking place in deltas. The dense growth 



