352 



DENUDATIONS. 



lakes, and those which empty themselves in the gulf of Mexico, are 

 only separated at their sources, by elevations not exceeding a few 

 feet, and when swelled by rain, the northern and southern rivers 

 sometimes interlock. In this plain there are no mountains. These 

 lakes were probably formed by partial subsidences, at the epoch 

 when the whole country was upheaved from the ocean. The ef- 

 forts of elevation and depression have been described in the prece- 

 ding chapter. 



Transversal vallies, or those which cut through mountain ranges, 

 nearly at right angles to the direction of the ranges they intersect, 

 may have been originally fissures or openings, made either at thp 

 period when the ranges were elevated, or subsequently, by the 

 same causes that have rent and displaced the secondary strata. 

 These fissures may have been afterwards widened by the erosion 

 of water. 



Geologists seem now generally agreed, that the action of rivers is 

 not sufficient to explain all the phenomena of valleys, and still less 

 to account for the fragments of rocks scattered over extensive plains, 

 at an immense distance from Alpine districts, where rocks similar to 

 these fragments occur. Another phenomenon, of more importance, 

 is altogether inexplicable by the action of rivers. Immense tracts of 

 the secondary strata, several hundred feet in depth, have in some dis- 

 tricts been torn off, and the materials entirely removed, except de- 

 tached patches, which here and there form isolated caps on distant 

 hills ; and incontestably prove, that they were once parts of one con- 

 tinuous stratum or formation. Numerous instances of this might be 

 cited in our own island. It is probable that the beds of chalk that 

 form the north and south downs of Sussex, once extended over the 

 Wealden beds. See p. 192. This local disappearance of a stra- 

 tum or formation, has properly been called Denudation. The the- 

 ory advanced by Mr. Farey, to explain these denudations, was, that: 

 the surface had been broken and swept away, by the near approach' 

 of a comet. But the most rational explanation that can be offered, 

 is that which ascribes the effect to a mighty deluge, sweeping over 

 the surface of the globe, tearing off part of its crust, and transport- 

 ing the fragments into distant regions, or into the ocean. The case 

 is one which may be truly said to be dignus vindice nodus, and the 

 geologist is compelled to call in the aid of Neptune ; for none of the 

 causes in present activity (however we may imagine them to be in- 

 creased in power or magnitude.) will be found adequate to produce 

 the denudation of an extensive district, and the disappearance of the 

 stony materials, by which it was covered. 



The fourth theory, which attributes the formation of valleys to the 

 sudden retreat of the sea from our present continents, is founded on 

 the admitted fact, that the sea has once covered them ; and wheth- 

 er we suppose that the bed of the ocean was deepened in one part 

 by a sudden subsidence, which drew off the water from another part; 



