Cit. XIV.] 



SUBTERRANEAN MOVEMENTS. 



315 



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 

 emerge. Nor have the regions now motionless Wt, n .lw* 



at rest ; and some of those which are at present the theatres 

 of reiterated earthquakes have formerly enjoyed a long con- 

 tinuance of tranquillity. But although disturbances have 

 ceased after having long prevailed, or have recommenced 

 after a suspension for ages, there has been no universal dis- 

 ruption 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 horizontally now retained 

 by some of the 



most 



^ 



wide areas. 



That the subterranean forces have visited different parts of 

 the globe at successive periods, is inferred chiefly from the 



groups of different 



ma 



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

 shire, we find the slaty beds of the ancient Silurian system 

 curved and vertical, while the beds of the overlying carboni- 

 ferous shale and sandstone are horizontal. All are agreed, 

 that in such a case the older set of strata had suffered great 

 dislocation 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 sud- 

 den or continuous lateral pressure. On the other hand, the 

 more ancient or Silurian group suffered only a local derange- 

 ment, and neither in Wales nor elsewhere are all the rocks of 

 that age found to be in a curved or vertical position. 



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 dis- 

 trict of Canada and the United States. They 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 the very rocks of which those mountains are 

 composed have been formed. 



