428 PROF. J. LOGAN LOBLEY, F¥.G.S., F.R.G.S., ON 
This considerable change of the course of a river in 
geologically recent times is an illustration of what great 
alteration may take place in river courses where the surface 
is approximately level and where olian deposits are 
accumulating. These deposits of blown sand are in the 
Landes very considerable and would in the past rapidly fill 
up any river channel from which the waterflow had been 
diverted. 
It would thus appear that after this region had subsided 
to avery considerable extent from its previous maximum 
elevation, but before it had reached its present levels, a 
great river, taking to the sea all the waters from the elacial 
ice of the mosthera side of fully half of the entire range of 
the Pyrenees, and draining besides the lower area between 
these mountains and the higher lands of mid France, flowed 
directly towards the head of the deep bay previously formed 
by glacial action. This large and rapid river would be a 
sufficiently powerful eroding agent to cut down a great 
gorge at the head of the glacial bay. The River Bidassoa, 
coming from the elevated mountain valleys, would also pour 
into the same bay its abundant waters and add its erosive 
power, while the Rio Urema, the Rio Bilbao, and other rivers 
from the Cantabrian mountains would indent by smaller 
ravines the sides of the great embayment. 
Besides the powerful erosive agents, glacial ice and rapid 
rivers, there must be added the waves of the sea, by which 
the sides of the great gorge in the coast-line would be cut 
back. By this action a widening process would contribute 
in giving the form and dimensions characterizing the Fosse 
de Cap Breton. 
When by continued subsidence still lower levels had been 
reached, approximating to those now existing, and when the 
glacial ice of the Pyrenean region had ceased to lie on the 
mountain slopes and the inland waters had much diminished, 
with a corresponding great diminution of momentum of 
flow, a tendency to deviation from a short, straight. and 
direct course would, with accumulations of Quaternary 
deposits, favour a division of the main stream, and so bring 
about a separation of the old one great river-system into two 
river-systems. Some of the head waters, and these the 
more western, would unite into a main stream taking a more 
southerly course to the sea; and others, and these the more 
eastern and the greater in number and volume, would unite 
into a main stream taking a more northerly course. Thus 
