Principles of Geology. 19 



tent, actually include the most striking variations in the surface of the 

 earth ; for it is certainly true, that the great mountain ranges which 

 seem to compose the skeleton of the earth ; the wide oceans, plains, 

 and level tracts, and even the remarkable lines of secondary hills 

 and most extensive vallies, are placed in accordance to the interior 

 structure of the earth. Hence, it follows that we must limit our in- 

 quiry, as to the changes produced on the surface of the earth by the 

 deluge, to the vallies and hills which seem evidently to have derived 

 their peculiar features from currents of water, since the consolidation 

 of the strata. Even thus limited, the subject is ample, fertile, and 

 instructive. Many vallies in a secondary country are excavated 

 through several strata, as limestone, clay, and sandstone, which ap- 

 pear on the opposite sides in most exact agreement as to thickness, 

 composition, and mode of arrangement. That such rocks were ori- 

 ginally deposited in continued planes, and, therefore, once connected 

 across the chasm or valley which now divides them, can hardly be 

 doubted. The vallies themselves bear marks of their origin ; their 

 bottom is a continued plane ; their sides correspond with answering 

 sinuosities ; and their every peculiarity suggests the action of decur- 

 rent water. From the time of Pythagoras to the present day, every 

 unprejudiced observer of nature has concluded that such vallies were 

 cut out of the planes of the consolidated strata, through one, two, or 

 more rocks, according to the depth of the excavation, and in this or 

 that direction, according to the facility with which the materials were 

 abraded. These are called vallies of denudation, and they are very 

 numerous and extensive. In western l^orkshire, the great mining 

 vallies of Teesdale, Swaledale, Yoredale, and Wharfdale, are mag- 

 nificent examples, and strongly impress the mind with the power of 

 the currents which occasioned them. In the eastern part of the 

 county, .the vallies of the Derwent below Malton, Rievaulx and Bils- 

 dale above Helmsley, Newton Dale above Pickering, and Hackness 

 near Scarborough, are remarkable and beautiful instances. 



There is one circumstance of common occurrence, which yields 

 so absolute a proof that vallies were formed at periods subsequent to 

 the deposition of the strata, and is in itself so curious, that though 

 few will seek more satisfactory evidence than in each case each val- 

 ley furnishes, it deserves to be mentioned. Some valleys cross and 

 cut through vertical strata, which must necessarily have been at first 

 deposited nearly horizontal. Therefore, such vallies were not pro- 

 duced till after the displacement of the rocks. 



