694 STRATIGRAPHICAL GEOLOGY. — [Boox VI. 
Wales and the west of England. For a prolonged period the sea 
must have been excluded, or at least must have been rendered unfit 
for the existence and development of marine life, over the area in 
question. The striking contrast in general facies between the 
organisms in the Silurian and those in the Carboniferous system 
proves how long the interval between them must have been. 
The geological records of this interval are still only partially 
unravelled and interpreted. At present the general belief among 
geologists is that, while in the west and north-west of Hurope the 
Silurian sea-bed was upraised into land in such a way as to enclose 
large inland basins, in the centre and south-west the geographical 
changes did not suffice to exclude the sea, which continued to cover — 
that region more or less completely. In the isolated basins of the 
north-west a peculiar type of deposits termed the Old Red Sandstone 
is believed to have accumulated, while in the shallow seas to the 
south and east a series of marine sediments and limestones was 
formed to which the name of Devonian has been given. It is thus 
supposed that the Old Red Sandstone and Devonian rocks represent 
different geographical areas, with different phases of sedimentation 
and of life, during the long lapse of time between the Silurian and 
Carboniferous periods. 
That the Old Red Sandstone, at least, does represent this prolonged 
interval can be demonstrated by innumerable sections in Britain, 
where its lowest strata are found graduating downward into the top 
of the Ludlow group, and its highest beds are seen to pass up into 
the base of the Carboniferous system. But the evidence is not 
everywhere so clear in regard to the true position of the Devonian 
rocks. That these rocks le between Silurian and Carboniferous 
formations was long ago shown by Lonsdale to be proved by their 
fossils. But it is a curious fact that where the Lower Devonian 
beds are best developed the Upper Silurian formations are scarcely 
to be recognized, or, if they occur, ean hardly be separated from the 
so-called Devonian rocks. It is therefore quite possible that the 
lower portions of what has been termed the Devonian series may in 
certain regions to some extent represent what are elsewhere re- 
cognized as undoubted Ludlow or even perhaps Wenlock rocks. 
We cannot suppose that the rich Silurian fauna died out abruptly at_ 
the close of the Ludlow epoch. We should be prepared for the” 
discovery of Silurian rocks younger than the latest of those in 
3ritain, such as M. Barrande has shown to exist in his Ktage H 
(p. 689). The rocks termed Lower Devonian may partly represent 
some of these later phases of Silurian life, if they do not also mark 
peculiar geographical conditions of a still older period in Upper 
Silurian time. On the other hand, the upper parts of the Devonian 
system might in several respects be claimed as fairly belonging to 
the Carboniferous system above. 
The late Mr. Jukes proposed a solution of the Devonian problem, 
the effect of which would be to turn the whole of the Devonian 
rocks into Lower Carboniferous, and to place them above the Old 
