412 EFFECT OF RIVERS ON THE LEVEL OF CONTINENTS. 



and table-lands have been having one foot worn off their 

 surface, the declivities and river-courses have lost nine feet. 

 Let it be further assumed that one-tenth part of the surface 

 of a country is occupied by its water-courses and glens, while 

 the remaining nine-tenths are covered by the plains, wide 

 valleys, or flat grounds. Now, according to the foregoing data, 

 the mean annual quantity of detritus carried to the sea is 

 equal to the yearly loss of Q-^QQ of a foot from the general 

 surface of the country. The valleys, therefore, are lowered by 

 TgW of a foot, and the more open and flat land by 10 ^ 00 of 

 a foot." 



Mr. Geikie calculates in this manner that Europe would 

 disappear in little more than 4,000,000 of years. I cannot 

 altogether accept this conclusion, for when a river has less 

 than a given amount of fall, it ceases to excavate. Thus the 

 effect of the Nile is to raise, not to lower, the level of Egypt, 

 and most of our large rivers near their mouths act in a some- 

 what similar manner. As regards the higher districts, how- 

 ever, his data are perhaps not far wrong, and if we apply 

 them to the valley of the Somme, where the excavation is 

 about 200 feet in depth, they would indicate an antiquity 

 for the Paleolithic epoch of from 100,000 to 240,000 years, 

 which, though arrived at from perfectly different data, agrees 

 with the periods A and B in the calculation made by Messrs. 

 Croll and Stone. 



In addition to the causes already alluded to, there is at 

 least one other astronomical phenomenon, namely, the change 

 in the obliquity of the ecliptic, which must be taken into 

 account in considering the effects which cosmical causes may, 

 or must, have exercised on climate. The whole question then 

 is one, not only of extreme interest, but also of very great 

 difficulty, and we are not, I think, at present in a position to 

 estimate with confidence the effects on climate which may 

 have been produced by these various causes. 



