1S25.] of Alluvial and Diluvial Formations. 243 



some of the facts may be thought too unimportant to deserve 

 any notice. If, however, they should throw any hght on a dis- 

 puted subject, or should they in any way strengthen the chain 

 of evidence by which one of the most important inductions of 

 geology has been estabhshed, they will not be altogether with- 

 out their use. I have the honour to be, Gentlemen, 



Your most faithful servant, 



A. Sedgwick. 



Sect. 1. — Alluvial Deposits. 



All the principal vallies of England exhibit in their higher 

 portions occasional examples of nearly horizontal deposits of 

 eomminuted gravel, silt, loam, and other materials accumulated 

 by successive partial inundations. The nature of these alluvial 

 deposits and the cause of them are so obvious, that it is unne- 

 cessary to refer to particular instances. If we descend from the 

 hilly and mountainous regions, and examine the courses of our 

 rivers near their entrance into any widely extended plains, we 

 frequently find their banks composed of incoherent materials of 

 a new character. They are not made up of thin layers of com- 

 minuted matter formed by successive inundations, or of silt and 

 turf-bog accumulated in stagnant waters, but of great irregular 

 masses of sand, loam, and coarse gravel, containing throug-h its 

 mass rounded blocks sometimes of enormous magnitude. It is 

 at once evident that the propeUing force of the rivers is entirely 

 inadequate to the transport of such materials as these. We 

 may observe, moreover, that they are not confined to the banks 

 of the rivers, but spread over all the face of the country, and 

 often appear at elevations many hundred feet above the level of 

 any natural inundation. To such materials as these the term 

 diluvial (indicating their formation by some great irregular 

 mundation) is now applied by almost all the English school of 

 geologists. 



The rivers which descend from the western moors and unite 

 m the great central plain of Yorkshire, afford a succession of 

 beautiful illustrations of the appearances which have been just 

 described. While rolling from the mountain chains, and uniting 

 with their different tributary branches, they leave masses of 

 alluvial matter in every place where the form of the valley 

 admits of such a deposit : and after passing through the infe- 

 rior region and escaping through many ravines and gorges into 

 the great plain of the new red sandstone, they then rind their 

 way tlirough enormous masses of diluvial debris which often 

 mask the inferior strata through considerably extended tracts of 

 country. If we follow any of these rivers into the central parts 

 of the great plain, we may still find (with occasional inteirup- 

 tiouB) the diluvial detritus deBcendiug with the surface of the 



R 2 



