LAPSE OF TIME AS INDICATED BY CHANGE OF CLIMATE. 397 



Moreover, as Sir Charles has himself pointed out, a very 

 large proportion of the mud brought down by the river is not 

 deposited in the delta, but is carried out into the gulf. In 

 the "Antiquity of Man"* he refers to the above-given calcu- 

 lation, and admits that the discharge of water seems to have 

 been much underrated by the earlier experiments. Messrs. 

 Humphrey and Abbot, who have recently surveyed the delta, 

 also remark that " the river pushes along its bottom into the 

 gulf a certain quantity of sand and gravel, which would," they 

 suppose, " augment the volume of solid matter by about one- 

 tenth." This, of course, would greatly diminish the time re- 

 quired; but, taking into consideration the quantity of mud 

 which is carried out to sea, and which was not allowed for in 

 the previous calculation, Sir Charles Lyell still regards 100,000 

 years as a moderate estimate ; and he considers that " the 

 alluvium of the Somme containing flint implements and the 

 remains of the mammoth and hyaena," is not less ancient. 



Again, whatever cause or causes may have produced the 

 great change which has taken place in the climate of Western 

 Europe, there can be little doubt that this change indicates 

 a very considerable lapse of time. We are indebted to Mr. 

 Hopkins for a very interesting memoir on this subject. Among 

 the possible causes of change he discusses 



Firstly. A variation in the intensity of solar radiation. 



To this theory Mr. Hopkins sees no a priori objection; but 

 he does not feel disposed to attach much weight to it, because 

 it is " a mere hypothesis framed to account for a single and 

 limited class of facts, and unsupported by the testimony ot 

 any other class of allied but independent phenomena." 



It is, moreover, open to the objections stated with great 

 force by Professor Tyndall,-f who argues that the ancient gla- 



* Appendix to third edition, t Heat considered as a Mode of 

 p. 16. See also Geological Journal, Motion, p. 192. 

 1869, vol. xxv. p. 11. 



