A HISTORY OF CUMBERLAND 
mechanical action of wind and rain, and a host of minor causes, each 
taking its own share in the work of destroying the outer part of the 
rock surface. The function of the rivers is mainly that of carriers of 
the materials rotted from the surface of the land ; but they, too, exercise 
a certain amount of erosive power, and co-operate with the allied forces 
in the general work of lowering the surface. It is easy to realize the 
true function of a river if any one will observe the quantity of mud 
being carried seawards by the Eden at Carlisle after heavy rain in the 
upper part of the valley. All that mud being carried past by the river 
was, not so long ago, solid rock m siti. It has been rotted by the 
action of the weather, and now it has been stripped off that surface of 
the land, which is therefore lowered by the amount represented by the 
quantity held in suspension by the river. Small as that quantity may 
appear, one has to remember that the process of stripping off the surface 
of the rock is going on continually, and has been doing so, on the 
surface of what was land for the time being, from the earliest period 
known. It is solely to the prolonged continuance of this process that 
the carving of the valleys is due. 
The next general principle to be borne in mind is that no two 
sorts of rock yield to the same kind of attack quite at the same rate. 
Some appear capable of withstanding exposure for very long periods 
without seeming to be any the worse, while others waste appreciably 
in the course of a single lifetime. It is this differential rate of decay 
which is the chief factor concerned in producing even some of the 
larger physical features of the landscape. All rocks waste more or less ; 
but the rock that wastes at the most rapid rate will be the first to be 
lowered to sea level ; while the more durable rocks, whose surface is 
being lowered at a slower rate, soon attain to a relatively higher level, 
and are very much longer in wasting to the level of the sea than the 
rock which when first exposed stood up with them. 
Lastly, the reader must endeavour to realize that the processes to 
which the configuration of a country is due are by no means rapid in 
their operation ; but that, on the contrary, they act in general quietly, 
gently, and usually at rates so slow as to be imperceptible. We have to 
deal with effects that have been produced not within a century, or within 
a thousand centuries, but which have required periods of time too long 
for the human intellect to comprehend, and the immensity of whose 
length can only be compared to the almost infinite intervals of space 
with which the astronomer has to deal. 
(c) Leaving general principles, we may now pass on to consider 
their application to the district under notice: The reader of the fore- 
going section of this article will have noted that there have been three 
(or more) great piles of rock laid in succession one on another, upon 
what one may term the ‘foundation stones’ of the rocks of Cumber- 
land. For the present we must dismiss the present configuration entirely 
from our minds, and try to realize that there was at one time an extensive 
plain, formed by the edges of a vast thickness of Cambrian, Ordovician 
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