G, W. Biilman — On Underclays. 351 



IV. — Undeeclays : A Pkeliminary Study. 



By G. W. BuLMAN, M.A., B.Sc. ; 



Corbridge-on-Tyne. 



SINCE the advent, in 1840, of Sir Wm, Logan's paper " On tlie 

 Characters of the Beds of Clay immediately below the Coal 

 Seams of South Wales," underclays have generally been regarded 

 as representing the soils on which grew the vegetation now forming 

 the coal. I am not aware, however, that any definite and detailed 

 hypothesis has been put forward to account for their origin. The 

 superior interest attaching to the question of the origin of coal has 

 apparently overshadowed the less important underclay, and it has 

 been left to fit itself in with any particular hypothesis of the former 

 which happened to be in favour. At the same time, two general 

 views of its origin have been suggested which do not, however, go 

 into any detail as to the exact manner in which it has been brought 

 about. First, there is the opinion that underclays are terrestrial 

 accumulations, thus expressed by the authors of the '' Geology of 

 the Yorkshire Coal-field " : 



"There can be no doubt then that the underclays are old vegetable 

 soils, and that, unlike all the Carboniferous rocks we have hitherto 

 noticed, they were formed not under water, but on dry land" (p. 19), 



Prof. Hull, on the other hand, in his " Coal-fields of Britain," 

 expresses the opinion that underclays were laid down in water : 



"Now these underclays are distinctly stratified, showing that they 

 have been deposited under water" (pp. 66-7). 



Sir A. Geikie expresses a similar opinion in the following passage 

 from his " Text Book of Geology," and further indicates that they 

 were laid down in the shallowing water, just before it became a 

 marsh and was taken possession of by vegetation : 



" Where the clay was laid down under suitable circumstances, 

 vegetation sprang up upon it. This appears to have taken place 

 in wide shallow lagoon-like expansions of the sea, bordering land 

 clothed with dense vegetation" (p. 477). 



With regard to the first of these views it almost seems to be 

 implied that the underclay was formed in the same way as a modern 

 vegetable soil. But this interpretation may be dismissed at once, 

 for the underclay, although it may well have played the role of 

 foundation to the marsh, and received the roots of the vegetation, is 

 not in the least like a vegetable soil, for a modern vegetable soil 

 is formed of vegetable matter mingled with disintegrated rock, and 

 passes by gradual transitions through the subsoil to the rock below. 

 An underclay, on the other hand, is obviously an independent 

 formation, and not formed by disintegration in situ of the rock 

 below, from which it is separated by clear lines of demarcation. 



The chief indictment against both these views of the origin of 

 underclay is, that no example of the formation of clays at the 

 present day, either on land or in very shallow water, has been 

 brought forward. In fact, all we know about such fine grained 

 deposits as clay leads us to attribute them — as a general rule, with. 



