UNIFORMITY OF CHANGE 429 



movements in one direction has not been perpetual through- 

 out all past time. There have been great oscillations of 

 level, by which a surface of dry land has been submerged to 

 a depth of several thousand feet, and then at a period long 

 subsequent raised again and made to emerge. Nor have the 

 regions now motionless been always at rest; and some of 

 those which are at present the theatres of reiterated earth- 

 quakes have formerly enjoyed a long continuance of tran- 

 quillity. But, although disturbances have ceased after hav- 

 ing long prevailed, or have recommenced after a suspension 

 for ages, there has been no universal disruption of the 

 earth's crust or desolation of the surface since times the 

 most remote. The non-occurrence of such a general convul- 

 sion is proved by the perfect horizontality now retained by 

 some of the most ancient fossiliferous strata throughout wide 

 areas. 



That the subterranean forces have visited different parts 

 of the globe at successive periods is inferred chiefly from the 

 unconformability of strata belonging to groups of different 

 ages. Thus, for example, on the borders of Wales and 

 Shropshire, we find the slaty beds of the ancient Silurian 

 system inclined and vertical, while the beds of the overlying 

 carboniferous shale and sandstone are horizontal. All are 

 agreed that in such a case the older set of strata had suf- 

 fered great disturbance before the deposition of the newer or 

 carboniferous beds, and that these last have never since been 

 violently fractured, nor have ever been bent into folds, 

 whether by sudden or continuous lateral pressure. On the 

 other hand, the more ancient or Silurian group suffered only 

 a local derangement, and neither in Wales nor elsewhere are 

 all the rocks of that age found to be curved or vertical. 



In various parts of Europe, for example, and particularly 

 near Lake Wener in the south of Sweden, and in many parts 

 of Russia, the Silurian strata maintain the most perfect hori- 

 zontality ; and a similar observation may be made respecting 

 limestones and shales of like antiquity in the great lake 

 district of Canada and the United States. These older 

 rocks are still as flat and horizontal as when first formed; 

 yet, since their origin, not only have most of the actual 

 mountain-chains been uplifted, but some of the very rocks 



