212 Henry 0. Forbes — Denudation — Rain and River. 



Quite lately, Prof. E. W. Claypole, of Antioch College, Obio, 

 in connexion with the theory of an all-powerful ice-sheet, has drawn 

 attention to the well-known great Silurian escarpment, which bears 

 a similar relation to the Laurentian Highlands in the Peninsula of 

 Ontario, and northern part of Lake Huron, and consists of a range 

 of cliffs in some places 200 to 300 feet high. Prof. Claypole writes:^ 

 "It appears as if geologists who advocate the excavation of the 

 basins of the great lakes by the action of northern ice flowing off 

 the Laurentian Highlands are somewhat oblivious of the existence 

 of this escarpment. If the ice possessed the enormous eroding 

 power on rocks and cliffs so often attributed to it, it must certainly 

 have cut away and destroyed this gigantic barrier to its advance 

 before proceeding to scoop out deep basins to the southward." 



In discussing these points it is necessary to assume that the 

 relative elevation of different parts of the continent have remained 

 unchanged during and since the Glacial period. This is probably 

 not strictly true, and may be so far from correct in some cases as to 

 invalidate arguments based on the assumption. Unequal elevation, 

 great in amount, must have occurred in the west toward the close 

 of the Tertiary, and may have continued in progress in Glacial, or 

 even in post-Glacial times. 



V. — Denudation — Eain and Eiver. 

 By Henry 0. Forbes, Esq. 



DUEING a recent residence in Portugal I paid a visit in Feb. 

 1877, to Coimbra, and while standing on the tower of the 

 University, whence a magnificent view of the surrounding country 

 can be obtained, I was much struck by the immense accumulation of 

 sand deposited over a wide area on both banks of the river Mondego, 

 by whose margin the city stands. A considerable, though, com- 

 paratively speaking, a small quantity was of recent date, arid was 

 evidently brought down by the heavy rains in the months of No- 

 vember^ and December of the previous year, which had produced 

 destructive floods throughout the country, and had here greatly 

 threatened the low-lying parts of the town. I was informed that 

 every year a large quantity of new sand is spread out over the 

 valley ; but the shortness of my stay here precluded any attempt to 

 estimate the yearly additions to the fluviatile stratum. 



Since my return to England, however, I have fallen on some 

 interesting notes, geologically speaking, in various old Portuguese 

 works, which I send you, as they afford, I think, an approximate 

 estimate to the amount of denudation in a known period. 



"When the Alani overran the Peninsula in the fifth century, their 

 king Ataces captured from the Suevi the town named by the 

 Romans Conembrica — the spot now bears the name Condeixa a 

 Velha. It seems to have suffered so severely in the struggle that 

 the erection of an entirely new city was resolved upon on the mag- 



1 Canadian Naturalist, vol. viii., No. 4, p. 197. 



2 Cf. Meteorological Notes from Lisbon, Nature, vol. xvi. 1877, p. 265. 



