CHAP. V.] DEVONIAN PERIOD. 65 



small in comparison, though Lake Cheviot may have been 

 40 or 50 miles long. 



During the existence of these lakes volcanic activity was 

 rife, and immense sheets of lava were poured over the 

 country and interbedded with the lacustrine deposits, and 

 all this time it is probable that the lake bottoms were not 

 very far above the sea. 



Along the southern border of the central basin there is, 

 in fact, evidence that there must have been some alter- 

 nating movements of depression, or such differential move- 

 ments between the lake-basin and the dividing ridges of 

 land that the area of the former was sometimes invaded 

 by the sea. Thus, near Lesmahago, a few hundred feet 

 above the base of the red beds, there are shales containing 

 marine fossils of Silurian species, and again, near Car- 

 michael in Lanark, 5,000 feet above the base, there is a 

 band of shale which has yielded a Graptolite, an Orthoceras, 

 and a Beyrichia f orms which must have continued to exist 

 in the neighbouring sea, and which prove that the Silurian 

 fauna was still prevalent in that sea. This shale band, 

 intercalated between thick masses of red sandstones, shales, 

 and conglomerates, which are supposed to be of lacustrine 

 origin, is a proof that the lake-basins were at first isolated 

 from the sea by the ridging up of land-barriers, rather than 

 by any general elevation of the country. 



During the formation of the higher beds, however, we 

 may suppose that a general elevation took place, and that 

 the lakes came to have excurrent as well as incurrent 

 rivers, till in course of time, as the excurrent streams cut 

 their channels deeper and deeper, the waters of the lakes 

 were partially or completely drained off, just as the great 

 Tertiary lakes of North America were drained by the ex- 

 cavation of the Colorado canon. The country would then 

 present the aspect of a high and dry upland, formed of 

 lofty hill ranges separated by immense sandy plains, the 



