446 Jukes — On the Gorge of the Avon. 



resulting streamlets have washed off a large part of the softer Mesozoic 

 cover of the country and disclosed the old Palceozoic hills and ridges. 

 Many of these are formed of Carboniferous Limestone, which, in the 

 south of Ireland, has itself been worn down into valleys and plains. 

 The reason is that in the south of Ireland the limestone was never 

 protected by any Mesozoic cover, but has been subject to the dis- 

 solving power of the rain-water during the whole period. About 

 Bristol it was so protected for a great part of the time. Had it not 

 been so, the only hills remaining thereabouts at the present day 

 would probably have been those of Old Eed Sandstone and the un- 

 destroyed Mesozoic outliers. 



The whole of England may, doubtless, have stood at a lower level 

 than it does now during part of the time that this process has been 

 going on, as well as at a higher level. When it was at a higher 

 level the action was accelerated and extended ; when it was at a 

 lower, it was retarded on the part that remained dry land, and 

 entirely stopped for all the ground that sank below the sea. Sink 

 England now 600 feet, you make an island of Wales, and a cluster 

 of islets of the rest of the country. On its re-elevation you might 

 find beds of sea- shells on that which had been the sea-bottom, and 

 sea- worn crags on the sides of the straits that had connected Bristol 

 and Liverpool. That, however, would be no proof that the sea 

 eroded those straits. It would be nearly as reasonable to suppose 

 that a canal was excavated by the water that lies in it. The sea 

 would waste the coasts doubtless, and deposit the materials at the 

 bottom of the straits, and on the re-elevation of the country the rivers 

 would have to set to work to scour out their old channels and the 

 rain to wash the valleys clean again. 



While looking into De la Beche's Manual for his description of 

 Clifton, I caught sight of a passage in which, quoting from a French 

 author, he enforces the impossibility of the Meuse having cut its 

 own channel through the hills of the Ardennes, because in the 

 higher parts of its course it runs over ground much lower than the 

 Ardennes, where it is only separated from the Seine by hills of a 

 hundred feet or so in height. This lower ground is formed of the 

 Oolites, with the New Eed rising gently from underneath them on 

 the east. The Moselle also runs across that country on its way from 

 the northern slopes of the Vosges to Treves, where it turns and cuts 

 by deep winding channels through the much loftier ground of the 

 Eifel and the Hundsruck. These, like the Ardennes are made of 

 siliceous slate-rock. 



The hypothesis of atmospheric erosion equally applies here. The 

 Oolitic, perhajDS even Cretaceous, plateau over which all three rivers 

 orginally ran, had a surface higher than that part of the Ardennes 

 and the Eifel, across which the rivers Meuse and Moselle took their 

 course. The rivers have ever since been steadily cutting their chan- 

 nels deeper and deeper along the courses they first selected, and the 

 districts adjacent to them have been lowered by atmospheric waste in 

 proportion to the depth the rivers cut down to, and the difference in 

 the nature of the rocks they cut through in different places. Where 



