DIASTROPHISM AND THE FORMATIVE PROCESSES 327 



of this range was characterized, not only by sharp folding, but by 

 some of the most remarkable overthrust faulting of which we have 

 any record. The horizontal thrusting amounts to many miles. 



According to Lake and Rastall this folding occurred during the 

 Devonian,^ since it followed the deposition of a portion of the Old 

 Red Sandstone. In Wales this portion passes downward con- 

 formably into the Silurian. Sir Archibald Geikie likewise stated 

 that the Old Red Sandstone of Britain consists of two divisions, the 

 lower of which passes down conformably into the Upper Silurian 

 deposits, while the two divisions are separated one from another 

 by an unconformity which makes a great physical and paleon- 

 tological break. ^ This break presumably corresponds to the 

 mountain-building. 



But Jukes-Browne, writing in 191 1, states that some beds which 

 were formerly called Old Red Sandstone are now placed in the 

 Silurian, while the mass which was formerly called "Middle Old 

 Red Sandstone" is now admitted to be the true Lower Old Red, 

 and known from its fish fauna to belong to the Lower Devonian.^ 

 The movement in Wales he places at the beginning of the Devo- 

 nian,'* while the principal physical features of Scotland had been 

 developed between the close of the Ordovician and the opening of 

 the Devonian period. In the southern uplands, where the time 

 limits are more closely drawn, the plication follows the Silurian, 

 for that system is included in the crumpling. The time of deforma- 

 tion is finally summed up by Jukes-Browne: "Thus the first upKft 

 of the outer ranges may have taken place in Silurian time ; the date 

 of the central axis we know [apparently close of Silurian] and 

 the final intense development of the pressures in the outer ranges 

 may have occurred either at the same time or even during the 

 actual formation of the lowest Old Red Sandstone. "^ 



The Scandinavian portion of this great mountain system appar- 

 ently affords less definite evidence of the precise time of this dis- 

 turbance, but Haug concludes that although we cannot, in the 



' Philip Lake and R. H. Rastall, Textbook of Geology (1910), pp. 342-43- 

 = Sir Archibald Geikie, Textbook of Geology, 4th ed., II (1903), 1006-7. 



3 A. J. Jukes-Browne, The Building of the British Isles (191 1), p. 114- 



4 Ibid., p. 119. 5 Op. cit., p. 126. 



