AND THE AVON GORGE. 
185 
outlier) would pass some 2 or 3 hundred feet over his head. 
That the Lias itself rested directly on the Palaeozoic rocks is 
shown by the fact of sheets of it still stretching across the 
Carboniferous Limestone to the N.E. of Durdham Down, still 
resting in patches on the Backwell hills to the W. of D undry, 
and on that of the Mendips in the neighbourhood of Harptree. 
“ The Lias then formerly reposed on the Carboniferous 
Limestone of Clifton Down, and the Oolite spread over that. 
The Severn and its tributaries flowing over this Oolitic plain of 
course cut channels in it. The original form of the surface was 
such as to turn the Avon towards the Severn instead of towards 
the Thames. The course it originally took it has ever since 
maintained, cutting down through the horizontal Mesozoic cover, 
and through the Palaeozoic rock it found underneath, in what- 
ever position it might lie, or whatever materials it might 
be composed of.” 
We have here, in fact, a conspicuous example of the 
differential action of general and special denudation. The 
Limestone ridge of the Downs is like the Quartzite band in our 
experimental illustration. It has effectually resisted that 
wasting action which has removed the Mesozoic cover. The 
Clifton Gorge is no chasm rent in the rocks. It is in no wise 
connected with any supposed volcanic upheaval in the Mendips 
or elsewhere. But it is the result of that continuous fretting of 
the stream over its rocky bed, which gives rise to special 
denudation, and which I have likened to the action of a file. 
Nor is this result any whit more remarkable, from a geological 
point of view, than the vast amount of general denudation which 
has removed so great a mass of Mesozoic rock from the country 
between Bath and Bristol. 
l.—^The Clifton Fault. 
The strata of the Avon Gorge do not present an unbroken 
