TRANSACTIONS OF SECTION C. 747 



each group beiutr composed of sandstone, followed bv shale, shale followed by 

 limestone. Similarly the Coal-measures present repetitions of sandstone followed 

 by shale, shale by coal. Limestone in the one case and coal in the other are 

 therefore comparable in the respect that each represents an episode when sedimen- 

 tation had come to a pause. Early views as to the orij^in of coal, namely, that it 

 was formed of vegetable matter drifted beyond the region to which the iinest 

 mineral sediment could reach, were in accordance with these facts. 



More_ minute examination of the strata, however, revealed proofs of land- 

 surfaces in the Coal-measures, and it was generally accepted that the coal-seams 

 represent forests in the place of their growth. The evidence may be summarised 

 as follows: — 



1. Rain-pittings, sun-cracks, and footprints prove that the surfaces of some of 

 the beds were exposed to the air. 



t.'. Erect tree-trunks of large size, in some cases attached to laro-e spreading 

 roots, are not uncommon. Land-shells, millipedes, and the skeletons of air- 

 breathing reptiles have occasionally been found within the hollow trunks. 



3. The underclays of coal-seams are traversed in all directions by branching 

 rootlets, unlike the drifted fragments in the bedding planes of the other strata" 

 Ihey were described as an invariable accompaniment of coals, and as bein<' 

 the soils in which the coal-forest was rooted. '^ 



4. Coal-seams, with thin minute partings, persist over vast areas, and it was 

 thought impossible that so wide and regular a distribution of the vegetable matter 

 could have been accomplished by drifting. 



5. The chemical composition of the coals was believed to prove that the ve^re- 

 table matter underwent partial decomposition in the open air before bein? 

 submerged or buried. 



This evidence, however, though it proves the existence of land surfaces, is not 

 conclusive of the coal-seams being forests in place of growth. The rain-pittings 

 sun-cracks, and footprints occur, not in the coals, but in the intervening strata. 

 Ut the erect tree-trunlcs a large proportion occur in sandstones devoid of coal, a 

 lew only having been found to stand upon an underclay, or to be associated with 

 seams ot coal. Vast areas of coal have been worked without any such trunks 

 having been encountered. The majority of the trunks, moreover, are destitute of 

 spreading roots, and are believed to have been floated to their present positions. 

 1 he land-shells, insect and reptilian remains, are of extremely rare occurrence. 



Ihe underclays do not resemble soils, inasmuch as they are perfectly homo- 

 geneous, and he with absolute parallelism to the other members of a stratified 

 series. Ihey are not always present beneath coal-seams, but, on the other hand, 

 often occur in them or above them. Frequently thev have no coal associated with 

 them.^ Ihe rootlets in them have no connection with the coal, which is a well- 

 stratined deposit with a sharply defined base. 



The persistence of the partings and characters of the coal over wide areas is in 

 favour ot their being subaqueous deposits, for on so large an excanse of land there 

 must have been river-systems and variations in the vegetation,' The stream-beds, 

 Jinown to miners as ' wash-outs,' are not proportioned in size to the supposed land- 



SUl ItlCGS, 



Sub-aerial decomposition of part of a mass of vegetable matter would take place 

 whether It were floating or resting on dry land. Spores, which enter largely into the 

 composition of many coals, would travel long distances either by wind or water. 



.Some coal-seams show clear proof of a drifted origin, as, for example, those 

 which are made up of a mass of small water-worn chips of wood or bark. Other 

 seams pass horizontally into bands of ironstone, and one case has been observed 

 ot a coal changing gradually into a dolomitic tufa, doubtless formed in a stagnant 

 lagoon, rutting aside exceptional cases, the sequence of events which preceded 

 t.ie deposition of a normal coal-seam seems to have been— firstly, the outspreadinP- 

 of sand or gravel with drifted plant-remains, followed by shale as the currenti 

 lost ^ elocity. J he water was extremely shallow, and even retreated at times, so 

 as to leave the surface open to the air. The last sediments were extremely fine, 



