170 L. Dudley Stamp — Limit hetiveen 



the greater part of the present Bassin de Dinant — and gave rise to 

 well-marked basal conglomerates and sandstones (Poudingue de 

 Fepin and Arkose de Haybes). The movement was isostatic in 

 its nature, and was coincident with a shallowing of the pre-existing 

 Upper Ludlow sea in North-East France. A very similar isostatic 

 movement seems to be indicated in South Wales as noted on the 

 correlation table. Thus in North-East France, Shropshire, and 

 certain districts of South Wales, although there is a gradual transition 

 from the Upper Ludlow to the shallower-water deposits of 

 Downtonian age, the most widely spread disturbance, heralding the 

 advent of the Devonian period, occurs in Lower Gedinnian and early 

 Downtonian times. The commencement of this movement may 

 possibly be indicated by the Ludlow Bone-bed. 



In Scotland the Downtonian rocks ^ are sometimes seen to rest 

 conformably on Upper Ludlow, with or without a conglomerate at 

 the base ; but an important transgression of the sea is indicated, 

 for example, in Kincardineshire,^ where Downtonian rocks, con- 

 formably overlain by Lower Old Eed Sandstone, rest on ? Upper 

 Cambrian. In the Southern Uplands there is sometimes a marked 

 discordance (Pentland Hills and Ayrshire), and sometimes apparent 

 conformity (Lanarkshire), between the Lower Old Red Sandstone 

 and the Downtonian. The folding in the district is so intimately 

 connected with the volcanic activity of Old Red Sandstone times, 

 that the area must be considered in relation to others rather than 

 by itself. 



From the standpoint of physical geology, surely the only logical 

 view is to regard the Downtonian as the commencement of the 

 Devonian Period. To regard it as Silurian is to admit the enormous 

 extension of the Silurian sea at the very end of the period, almost 

 everywhere to be succeeded imperceptibly by Devonian or Old 

 Red Sandstone, the rocks of the latter having, except in special 

 cases, no natural stratigraphical base. 



To this it may be objected that the Upper Gedinnian of North-East 

 France and Belgium, as well as parts of the succeeding Coblentzian, 

 are of " Old Red Sandstone " type, and are therefore to be regarded 

 as marking the close of a period rather than as an integral part of 

 the Devonian Period. Truly the rocks of this age in the Ardennes 

 are not rich in fossils, and consist frequently of sandstones, indicating 

 shallow-water conditions unfavourable to life. But lithology is 

 by no means a sure indication of lacustrine or true Old Red Sandstone 

 conditions. The discovery of marine fossils in the Downtonian of 

 England — including the Temeside Shales of Clun Forest — in rocks 

 which were regarded as typical of Old Red Sandstone, is paralleled 

 in Belgium in the work of Dr. E. Asselbergs. This geologist, well 



1 Silurian Bocks, Scotland (Mem. Geol. Surv.), 1899, pp. 68-9, p. 568 

 et seq. 



^ Campbell, " The Geology of Soutli-Eastern Kincardineshire " : Trans. 

 Eoy. Soc. Edin., vol. xlviii, 1913, p. 923. 



