60 



Royal Society :■ 



contains granite from the Morvan, the Oise 

 slate-rock debris from the Ardennes^ the Marne 

 nothing older than oohtic debris, whilst the 

 Therain and the Somme valleys contain nothing 

 but debris of the chalk and tertiary strata. 

 The same rule applies to the English area ; but 

 the fact is not so apparent, owing to various 

 conflicting elements pointed out by the author, 

 who shows, by a map of the two countries, 

 how great are the range and spread of these 

 beds, and how large a proportion of our drift- 

 gravels are of fluviatile origin. 



The High-level Gravels. — From the facts re- 

 corded by the several independent observers 

 abroad, and from his own observations in this 

 country, the author arrives at a general proposi- 

 tion illustrated by the accompanying diagram, 

 which shows, — 1. D, a major valley or plain of 

 denudation anterior to the excavation of the river- 

 valley. 2. e, a non-fossiliferous drift on the slope 

 and base of D. 3. C, the river-valley. 4. ^, the 

 high- and c, the low-level gravels. 5. a, recent 

 alluvium. 6. A, the present river-channel. 



The high-level gravels (d) appear on both sides 

 of the valley, and their connexion before the ex- 

 cavation of C is pointed out. This is one of the 

 points insisted upon by the author; the two 

 having been generally considered as contempo- 

 raneous, or even sometimes the higher-level de- 

 posits as newer than those of the lower level. It 

 is to be observed that the phenomena here re- 

 ferred to relate to broad valleys, and not merely 

 to river-channels. 



The loess is not shown in the diagram, other- 

 wise the section represents the condition of the 

 case on the supposition that all the parts are 

 complete. But this rarely happens. Some low- 

 level gravel is constant, but the high-level gravels 

 are only occasionally preserved. Sections are 

 then given to explain the cause of their absence 

 — such as where the valley C being wider than 

 the original bed of the old river which deposited 

 the gravel c?, the latter has been necessarily alto- 

 gether removed. 



That the formation of the higher gravels can 

 be owing to the action of the present rivers is 

 clearly impossible under existing conditions ; for 

 not only are they far above the level reached 

 by the rivers at the highest floods, but also 

 the sectional area of the valleys, compared to 



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