COAL : ITS STRUCTURE AND FORMATION. 357 



gradually come to be distrusted as inadequate to explain 

 certain facts. 



In 1842 Logan drew the attention of geologists to the 

 constant occurrence in the South Wales coalfield, and in 

 other districts, of a characteristic unstratified argillaceous 

 rock underlying each bed of coal. It was shown by this 

 observer that the underclay not only occupied this constant 

 position, but that it was further characterised by containing 

 numerous stigmarian remains. The idea of an old surface 

 soil was gradually accepted as the most satisfactory inter- 

 pretation of these facts. It was held that the beds of coal 

 had been produced from a thick mass of vegetable debris, 

 which had accumulated during centuries of forest growth on 

 the underclay surface soils ; and in the stigmarias were 

 recognised the forked roots of sigillarian stems which had 

 largely contributed to the substance of the overlying coal. 

 Bowman and others gave strong support to this view, and 

 the former, in his theory of intermittent and irregular sub- 

 sidence, found a convenient means of explaining certain 

 peculiar features in the arrangement and relative positions 

 of seams of coal. 



During the last fifty years there have been numerous 

 writers who have warmly advocated the theory that the coal- 

 forming materials accumulated on the surface of forest- 

 covered areas, and that, after subsidence had brought about a 

 general submergence, the vegetable remains became sealed 

 up under a covering of mud and sand. 1 Leaving out ol 

 account any differences in detail, the general concession of 

 opinion has, until recently, been strongly in iavour of this 

 so-called growth-in-place theory of the formation of coal. 

 It has come to be regarded as the orthodox standpoint from 

 which to explain most conveniently the facts of Upper 

 Carboniferous stratigraphy. We may briefly summarise 

 some of the main arguments quoted in support of the 

 growth-in-place views : (1) The almost constant occur- 



1 An interesting account of coal building on drift theory lines will be 

 found in Coal (see bibliography). Hull devotes a chapter of his book on 

 the British coalfields to this subject. 



