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ILLUSTRATIONS OF THE 



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nal inequalities of the globe, becaufe we can 

 trace it no farther back), and that this hollow 

 in the courfe of ages, has been filled up by the 

 gravel and alluvial earth brought down by the 

 river, which is now cutting its channel through 

 materials of its own depofiting. There is no 

 great river that does not afford inflances of this, 

 both in the hilly part of its courfe, and where 

 it defcends firil from thence into the plain. 

 Were there room here for the minuter details 

 of topographical defcription, this might be il- 

 luftrated by innumerable examples. 



317. it is faid above, that the water muft 

 have run or ftood, in former times, as low as the 

 prefent bottom of the river ; but there is often 

 clear evidence, that it has run or flood much 

 lower, becaufe the alluvial land reaches far be- 

 low the prefent level of the river. This is known 

 to hold in very many inflances, where it has 

 happened that pits have been funk to confider- 

 able depths on the banks of large rivers. By 

 that means, the depth of the alluvial ground, 

 under the prefent bed of the river, has been dif- 

 covered to be great ; and from this arifes the 

 difficulty, fo generally experienced, of finding 



good foundations for bridges that are built over 

 rivers in large vallies, or open plains, the ground 

 being compofed of travelled materials to an un- 

 known depth, without any thing like the native 



or 



