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 urrangement. ‘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 3 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 Yorkshire, 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 
“ounty, the vallies of the Derwent below Malton, Rievaulx and Bils- 
ale above Helmsley, Newton Dale above Pickering, and Hackness 
near Scarborough, are remarkable and beautiful instances. 
“Here is one circumstance of common occurrence, which yields 
80 absolute a proof that vallies were formed at periods subsequent to 
the deposition of the strata, and is in itself so curious, that though 
ew 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. , 
