1893.] evidence for the Recurrence of Ice Ages. 229 



Sometimes they are broken and the pieces separated. Rolled 

 balls of boulder clay with pebbles and shells stuck on them get 

 buried with the rest. 



Exactly the same process is now going on along the coast of 

 North Wales, and an exactly similar deposit is the result\ Dead 

 shells of species that live along the shore, of those that haunt the 

 rocks, of those that burrow in the mudbanks, and of those that 

 frequent the deeper waters are all thrown up together. 



The shells of the Moel Tryfaen, Colwyn, Clwyd, or Minera 

 terraces are not arctic, though there are some of more northern, 

 say Scandinavian, aspect than now inhabit our coast, and, what 

 seems to be a conclusive refutation of the Ice-push theory, the 

 facies is different in the different terraces. On the extreme glacial 

 theory, the ice must have crept across the temperate sea and 

 pushed in front of it, once for all, the remains of all the forms of 

 life that could not escape and then left some at one level and 

 some at another on adjoining parts of the coast. It could not go 

 back to fetch any more, and, if it did recede in parts, surely the 

 forms of life would then be more arctic in character than we 

 find in these marine terraces. 



The West Side of the Atlantic Basin. 



Now if we turn to the other side of the basin we shall find 

 the same kind of evidence of enormous alternate upheaval and 

 depression. We are not now concerned with the causes of the 

 movements but only with their recurrence and their extent. 



The Uinta Mountains^ rise as a single great fold and though 

 80,000 feet of the upHfted strata have been swept away, they still 

 rise some 10,000 feet. The upper Palaeozoic and Mesozoic are, 

 according to Powell, 30,000 feet thick. The vast scale of the 

 movements is further shown by the fold being cut off on its 

 northern and steeper side by a fault of 20,000 feet downthrow. 

 As in the case of the Sierra, where after a gradual ascent to a 

 height of 15,000 feet, there is a sudden drop on the East with a 

 fault-cliff nearly 11,000 feet high. The primary fold of the 

 Sierra was formed towards the close of the Jurassic period, but 

 the great uptilts of its eastern side are referred to an age as late 

 as the end of the Tertiary period. 



The thickness of the strata involved in these great folds is im- 

 mense. According to Clarence King the Palaeozoic and Mesozoic 

 strata of the Wasatch are about .50,000 feet thick. The Trias of 

 Western America is sometimes as much as 15,000 feet thick. 



1 See above, p. 222. 



■- Le Coute, Journ. GeoL, i. 1893, pp. 547, 553: Clarence King, Report Geol. 40«" 

 Parallel, Vol. i. 



