242 STRATIGEAPHICAL GEOLOGY 



western parts of the continent ; consequently we must suppose 

 that either the subsidence was greater in the east than in the west, 

 or that there were larger areas of low-lying land in the east. The 

 first hypothesis seems the most probable because the lacustrine 

 area of South Wales lay so close to the border of the continent that 

 we can hardly imagine it to have been at a high level above the 

 sea, nor is the intervening isthmus likely to have been very high 

 along its whole length. 



The Orcadian basin is indicated on the map (Fig. 80), 

 although we have no proof of its existence as a basin of deposition 

 during Lower Devonian time. It may have existed in the form of 

 a wide valley which, by subsequent disturbances and alterations of 

 level in the northern part of the continent, was converted into a 

 large lake-basin. In any case we must suppose that the river 

 which ran out of this lake opened somewhere into the Middle 

 Devonian Sea, for it can only have been by means of the ex-current 

 river that the fish made their way into the lake, ascending it as 

 salmon do rivers at the present day. Whether this river flowed 

 eastward or northward is the question, but unless it traversed the 

 range of mountains which certainly then existed in Scandinavia 

 it must have taken a northerly course into a northern sea. 



The lapse of time indicated by the slow accumulation of the 

 Middle Devonian shales and limestones must have been very great, 

 and it must not be estimated by the small thickness of such 

 deposits in Brittany and Cornwall, but by their maximum develop- 

 ment, which is probably that to be found in the Ardennes and the 

 Rhenish provinces (see p. 215). 



Again, it is noteworthy that if we had nothing but the succession 

 of marine deposits as a record of the Devonian period, we should 

 unquestionably have supposed that the whole period was one of 

 quiet and uninterrupted subsidence, and that the epoch of the 

 Middle Devonian merged into that of the Upper Devonian without 

 any great physical change. 



The terrestrial deposits, however, furnish us with evidence of a 

 great change, though it is quite possible that this was accomplished 

 without any great surface or subterranean disturbances. The 

 change probably involved some movement of the earth's crust, 

 but this can hardly have been elevation, for it ushered in the 

 subsequent subsidence of Carboniferous time ; neither was it 

 immediate subsidence, or we should have had marine beds in the 

 place of the Kiltorcan Beds and Upper Old Red of Wales. Rather 

 it seems to have been a change in the climate and in the terrestrial 

 conditions of deposition. 



It is possible that toward the close of Middle Devonian time 



