TRANSACTIONS OF THE SECTIONS. 63 
the sides of the ravines. He will see the marks of the old cataracts that once 
fell over these ledges, but which now are removed to other places, or converted 
into mere rapids, or perhaps altogether obliterated by the cutting down and cutting 
back of the streams. Torrs and pinnacles will be left here and there, perhaps, 
rising up from the bed of the stream, showing the former islets and rocks which 
resisted the erosive action better than the parts on each side of them. Where a 
softer and more yielding mass of rock occurred, there the glen widens into an open 
valley ; the narrowest and most jagged and steep-sided glens are just where the 
rocks are most hard and intractable, and best calculated for resisting the chemical 
and mechanical action of running water. 
The scale upon which these operations have been carried out does not affect the 
nature of the argument. The action has been the same in the miniature glens of 
our own mountains and in the grander and more awful abysses that gash the sides 
of the Alps, the Andes, and the Himalayas. 
In all cases when the river comes down now, or has formerly come down, in the 
form of a glacier, before springing into running water, the ice-mass has of course 
scooped out and deepened and widened the valley in its own peculiar fashion. 
When we leave the mountains and come down into the lower lands, where the 
rivers wind with a more gentle stream from side to side of broad open valleys, 
through wide alluvial flats, still it is to the river that the present form and depth 
of the valley are due. Whatever may have been the undulation of the original 
surface of marine denudation which determined the course of the primary stream, 
the river has long since cut down beneath that surface, and is still occupied in 
cutting deeper, so long as it retains any sensible current at all. It effects this by 
undermining the bank now on one side and now on the other side of the valley, 
shaving off a little corner here and another there, so that a river not a hundred 
ards broad, perhaps, may eventually form a valley of several miles in width. 
The obstructions it accumulates from time to time in its own bed constantly 
deflect its channel, so that ultimately it visits every part of the valley. 
In many cases the mere deepening of the valley may necessarily widen it also, 
since the rocks may be of such a composition, or may lie in such a way, as not to be 
able to form a bank of any steepness; and the materials, therefore, always slip down 
towards the bottom of the valley as fast as their bases are cut into. 
It is true that all these processes are infinitesimally slow; but if carried on 
through a period of time indefinitely great, it is obvious that it is impossible to 
assign a limit to the amount of their results. 
I have for several years been studying the origin of the river-valleys of the 
South of Ireland, and have, since the last meeting of this Association, been com= 
elled to arrive at the conclusion that the great limestone plain of the centre of 
‘ieee has lost a thickness of 300 or 400 feet at least, by the mere action of the 
rain that has fallen upon it. As a corollary of this conclusion, I have also been led 
to perceive that the longitudinal and lateral valleys of the Irish mountains—and if 
of them, then those of all other mountain-chains of the world—are the result of 
the action of the water or the ice that has been thrown down on them from the 
atmosphere. 
Tf we take any mountain-chain and its adjacent lowlands, and suppose no rain 
to fall upon them for a time, and that all the valleys of whatever description were 
filled up, and the sides of the mountains smoothed over from their peaks to their 
bases, 1 believe the surface thus produced would be one representing the limits of 
marine denudation ; then let rain begin to fall on such a country, and all the ela- 
borate structure of valleys, gorges, glens, and ravines would be produced by it. 
I believe that the lateral valleys are those which were first formed by the drain- 
age running directly from the crests of the chains, the longitudinal ones being sub- 
sequently elaborated along the strike of the softer or more erodable beds exposed on 
the flanks of those chains. Ido not, of course, intend to say that any country ever 
existed without valleys, since valleys of some kind must commence as soon as the 
first peaks of the mountains show themselves above the sea, and must be continued 
and extended in proportion to the extent of the land which gradually rises into the 
atmosphere. Atmospheric denudation and marine denudation have always been at 
work simultaneously upon the different parts of every land in the globe, and their 
