1819.] Greenough on the First Principles of Geology. 369 



Valleys usually widen as they advance ; but this is modified 

 by two disturbing causes. 1 . The breadth of valleys depends on 

 the comparative hardness of the substances which bound them. 

 Hence where the bounding substances become harder, the valley 

 may contract in its dimensions as it advances. 2. The size and 

 direction of a valley change as often as it is joined by lateral 

 valleys. 



From the phenomena of valleys, it is obvious that they have, 

 been formed by running water, and consequently mountains do 

 not owe their origin to volcanoes. 



It is a very common thing to observe a series of valleys 

 including each other. Thus the valley of the Thames at London 

 is contained in that of which Clapham Rise forms part of the 

 boundary on one side, and the Green Park on the other, and 

 this again is contained in the large valley between Highgate and 

 Sydenham. Arrived at these points, we find our horizon 

 bounded by a chalk ridge still loftier. 



Another phenomenon from which we derive the knowledge 

 that a debacle, or flood, has at one time or other sweeped the 

 present surface of the earth, is the vast quantity of alluvial matter 

 scattered in such profusion over immense tracts of country. These 

 matters have been derived from the breaking up of rocks situated 

 on a higher level than themselves. The largest masses are 

 found nearest the parent rock, and they diminish in size accord- 

 ing to their distance. The blocks and pebbles found at the 

 greatest distance are composed of the hardest and most inde- 

 structible materials. Substances breaking into cubic or hexa- 

 gonal blocks are found at a greater distance than those which 

 break into fragments with acute angles. Hence the reason why 

 granite is found at such a distance from the parent reck. 



It is obvious that the blocks of granite found on the surface 

 of alluvial tracts are too large to have been transported by ordi- 

 nary rivers ; neither can we ascribe the excavation of valleys to 

 the action of rivers ; for, 



1 . Some valleys are dry, and could not possibly have been 

 excavated by a river which does not exist. 



2. The source of the river is often balow the head of the 

 valley ; demonstrating that the valley existed before the river. 



3. The alluvial land often reaches far below the bed of the river. 

 In such cases it is evident that the river did not excavate the 

 Valley ; on the contrary, it has been employed in filling up the 

 valley. 



4. Both exterior and interior valleys cannot be ascribed to the 

 action of the same river. 



5. Rivers change their beds, and therefore are not fitted for 

 excavations. 



6. Rivers cannot be supposed to have formed their banks with- 

 out having been at one time without them — a supposition which 

 is absurd. 



Vol. XIV. N° V. 2 A 



