SIR CHARLES LYELL 215 



demonstrates that the persistency of subterranean movements in one 

 direction has not been perpetual throughout 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 earthquakes have for- 

 merly enjoyed a long continuance of tranquillity. But, although 

 disturbances have ceased after having long prevailed, or have recom- 

 menced after a suspension of 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 convulsion 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 unconformabil- 

 ity of strata belonging to groups of different ages. Thus, for ex- 

 ample, 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 suffered 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 horizontality ; and a similar 

 observation may be made respecting limestones and shales of like an- 

 tiquity 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 of which those moun- 

 tains are composed have been formed, some of them by igneous and 

 others by aqueous action. 



It would be easy to multiply instances of similar unconformability 



