346 



CHAPTER XXllL 



ON THE FORMATION OF VALLEYS, AND THE GEOLOGICAL THEORIEST 

 RELATING TO VALLEYS AND DENUDATIONS. 



On the Causes that have broken the Surface of the Globe. — Erosive Action of 

 running Water, illustrated by the Process called Hushing. — Bursting of Lakes. 

 — Some valleys originally formed by Elevation or Subsidence, and subsequent- 

 ly enlarged by the Action of Water. — Different Theories respecting the Forma- 

 tion of Valleys.— Theory of Werner— of Hut'ton.— Of Elevation.— Of the retir- 

 ing Waters of the Ocean. — Theory of Excavation and Denudation by Deluges. 

 Modification of this Theory by Sir James Hall; its Application to Explain De- 

 nudations, and Transportation of Blocks of Granite from the Alps. — Particular 

 Phenomena, presented by the scattered Blocks in the Vicinity of Geneva. — De- 

 nudation of Stratified Rocks, efifected by the same Causes which have broken the- 

 Primary Rocks, and scattered their Fragments into distant Districts. 



From what has been slated in the preceding chapter, respecting 

 the elevation and submersion of the earth's surface, the geological 

 student might infer, that such elevations and submergences offer a sat- 

 isfactory explanation of the formation of valleys, but the inference 

 would be erroneous. There are two distinct causes which have 

 modified the surface of the globe ; the one internal, dependent on the 

 atmosphere which surrounds it ; beside these, there is the ceaseless 

 flux and reflux of the ocean, dependent on the attractive forces of 

 the sun and moon, and on the earth's diurnal revolution on its axis. 

 The two former causes have been principally concerned in the for-, 

 mation of valleys ; and there are few valleys in which the combined 

 effects of both these causes may not be traced. The inequalities of 

 surface produced by the upheaving of mountain ranges, or the emer- 

 gence of continents from the ocean, must have originally determined 

 the course of the retiring water, or of atmospheric water precipitated 

 in rain. Of the power of atmospheric water, to act upon the sur- 

 face of the globe, we can form but a very feeble idea, from what 

 we observe in our own country. In warm climates, as much rain 

 will sometimes fall in one hour, as falls at different times, during, 

 three months, in northern latitudes : added to this, when the rain de- 

 scends in mountainous regions, the water is suddenly collected into 

 powerful rivers, rushing with incredible violence to the lower valleys. 

 At remote epochs, it is highly probable that many elevated depres- 

 sions, which are now mountain valleys in alpine regions, upheld the 

 waters and formed lakes, that have subsequently burst their barriers, 

 and have ploughed a passage for the succeeding rivers, when the 

 drainage of the country became more regular. 



To enable the reader to form some notion of the force of falling, 

 water, carrying with it loose stones that occur in its passage, it may 

 be useful to describe a process called hushing^ in Westmoreland ► 



