348 



VARIOUS FORMATION OF VALLEYS. 



er. The action of rivers in extensive and level valleys, tends rather 

 to fill them with debris, brought from the more elevated countries in 

 which the rivers had their origin, than to excavate them deeper. 



The formation of the greater number of valleys cannot be explain- 

 ed by the action of water alone. There are valleys of elevation 

 formed by the raising of the strata on each side — valleys of subsi- 

 dence, formed by the sinking of the ground, leaving the adjacent 

 rocks unmoved — valleys formed on tlie line of faults, in which the 

 rocks on one side have been thrown up or depressed — valleys of dis- 

 ruption, where a range of mountains, or an extent of country has 

 been rent by earthquakes or by subsidence. Most of the valleys 

 formed originally by these causes, have been subsequently enlarged 

 or modified by the action of water. There are, indeed, instances 

 of valleys and ravines formed entirely by the continued erosion of 

 water ; such is the valley of Niagara, between Queensiown and the 

 Falls. (See the frontispiece to the present volume.) Other instan- 

 ces might be cited, in which the action of water is equally evident. 

 In many cases, however, where water appears to have been the sole 

 agent in excavating rocks, I am inclined to believe, that an original 

 break or fissure has greatly accelerated the process. In many broad- 

 er valleys, the excavation must often have been effected by more 

 powerful agents than any which we perceive in present operation ; 

 and when a broad outlet is once made, the subsequent drainage of a 

 country may work its way to the sea in a very sinuous course; but 

 this sinuous course, does not prove that the valley had been originally 

 formed by the river that flows through it. 



Besides the action of mountain torrents, the bursting of lakes, and 

 the regular flowing of rivers, many geologists believe that the exca- 

 vation of valleys, and the transportation of loose rocks, have been 

 effected by the more powerful agency of the ocean, thrown over 

 the surface of the land by the great convulsions that have upheav- 

 ed mountain ranges and continents. For the benefit of the geolo- 

 gical student, I shall endeavour to give a brief outline of the princi- 

 pal theories that have been maintained respecting the formation of 

 valleys; but the first of these theories is now admitted to be un- 

 tenable. 



The formation of valleys has been ascribed to the following caus- 

 es : — 



1st, To the original unequal deposition of the earth's surface. 

 2d, To excavation, by the rivers that flow through them. 

 3d, To the elevation or subsidence of part of the earth's surface. 

 4th, To excavations, caused by the sudden retreat of the sea from 



our present continents, 

 5th, To excavations by inundations or deluges, that have sudden- 

 ly swept over the surface of different parts of the globe. 

 I shall notice the leading facts that favour or oppose each of these 

 theories. The disappearance of large portions of strata from districts 



