■30 Notices of Memoirs — Vegetation oj the Coal Period. 



The subsidence was counterbalanced during Coal-measure times 

 by sedimentation, for the occurrence of marine beds among deposits 

 of a generally estuarine aspect proves that the surface was maintained 

 at or near sea-level. The Carboniferous sediments consist, in the 

 majority of coalfields, of marine limestones in the lower part, of 

 marine grits and conglomerates in the middle part, and of estuaro- 

 marine sandstones and shales in the upper part. The sequence is 

 due, firstly, to the admission of the sea to the subsiding areas ; and 

 lastly, to the restoration of level brought about by sedimentation and 

 denudation. But there is evidence also of the sedimentation having 

 been more or less spasmodic. Thus the Limestone Series generally 

 consists of repetitions of small groups of strata, each group being 

 •composed of sandstone, followed by shale, shale followed by lime- 

 stone. 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 this respect, that each 

 represents an episode when sedimentation had come to a pause. 

 Early views as to the origin of coal, namely, that it was formed of 

 vegetable matter drifted beyond the region to which the finest 

 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 summarized as follows : — 



(1) Rain-pittings, sun-cracks, and footprints prove that the surfaces 

 of some of the beds were exposed to the air. 



(2) Erect tree-trunks of large size, in some cases attached to large 

 ■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. They were described as an invariable 

 accompaniment of coals, and as being 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 vegetable matter could have been accomplished by drifting. 



(5) The chemical composition of the coals was believed to prove 

 that the vegetable matter underwent partial decomposition in the 

 open air before being 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. Of the erect tree-trunks 

 a large proportion occur in sandstones devoid of coal, a few only 

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



1 C. Brongniart and others have shown that air-breathing insects of the orders 

 Neuroptera, Orthoptera, Thysanura, and Homoptera, were very numerous in the 

 Coal-period in Europe and America. — Edit. Geol. Mag. 



