Date of the Glacial and the Upper Miocene Period. 381 



determined by the rate at which the rocks are disintegrated and 

 decomposed, but by the quantity of rain falling, and the velocity 

 with which it moves off the face of the country. Every river- 

 system possesses a definite amount of carrying-power, depending 

 upon the slope of the ground, the quantity of rain falling per 

 annum, the manner in which the rain falls, whether it falls gra- 

 dually or in torrents, and a few other circumstances. When it 

 so happens, as it generally does, that the amount of rock disin- 

 tegrated on the face of the country is greater than the carrying- 

 power of the river-systems can remove, then a soil necessarily 

 forms. But when the reverse is the case no soil can form on 

 that country, and it will present nothing but barren rock. This 

 is no doubt the reason why in places like the Island of Skye, for 

 example, where the rocks are exceedingly hard and difficult to 

 decompose and separate, the ground steep, and the quantity of 

 rain falling very great, there is so much bare rock to be seen. 

 If, prior to the glacial epoch, the rocks of the area drained by 

 the Mississippi would not produce annually more material from 

 their destruction under atmospheric agency than was being carried 

 down by that river, then it follows that the country must have 

 presented nothing but bare rock, if the amount of rain falling- 

 was then as great as at present. 



No proper estimate has been made of the quantity of sediment 

 carried down into the sea by our British rivers. But, from the 

 principles just stated, w 7 e are warranted to infer that it must be 

 as great in proportion to the area of drainage as that carried 

 down by the Mississippi. For example, the river Tay, which 

 drains a great portion of the central Highlands of Scotland, car- 

 ries to the sea three times as much water in proportion to its 

 area of drainage as is carried by the Mississippi. And any one 

 who has seen this rapidly running river during a flood, red and 

 turbid with sediment, will easily be convinced that the quantity 

 of solid material carried down by it into the German Ocean must 

 be very great. Mr. John Dougall has found that the waters of 

 the Clyde during a flood hold in suspension -g-J- - by bulk of sedi- 

 ment. The observations were made about a mile above the city 

 of Glasgow. But even supposing (what is certainly an under- 

 estimate) the amount of sediment held in suspension by the 

 waters of the Tay to be only one-third of that of the Mississippi, 

 viz. 45V0 by weight, still this would give the rate of denudation 

 of the central Highlands at I foot in 6000 years, or 1000 feet in 

 6 millions of years*. 



But, after all, one foot removed off the general level of the 



* See a valuable paper by Mr. Archibald Geikie on " Denudation as a 

 measure of Geological Time," which will shortly be published in the Trans- 

 actions of the Glasgow Geological Society. 



