GEOLOGY 
of the lead veins also is very similar in this respect. The upper limits of 
both the dyke and the metalliferous veins roughly conform to the broad 
outlines of the present surface, as if the downflow of cold waters from the 
surface had checked the upward growth of the dyke, and had also cooled 
the hot springs, so that both the dyke and the mineral vein terminated 
upward at a lower level where there were depressions of the surface 
than where there were elevations. It is remarkable that this does 
not apply to the low ground of Edenside ; but the conformation of the 
surface there may well have been of more recent formation, and the 
depression may be due to the more rapid waste of the Cretaceous Rocks 
there faulted in, as compared with the surrounding rocks, which are of a 
more durable kind. 
(c) It was probably during late Tertiary times that the last upheaval 
along the Pennine Faults took place. Attention has more than once 
before been directed to the fact that this zone of weakness is one along 
which differential uplifts have been many times repeated—the earliest 
movements probably dating back to the period following the close of 
Silurian times. 
(2) Taking the aggregate thickness of all the marine limestones that 
have been formed since the close of the Cretaceous Period, and assuming 
that these have been formed at the rate of one foot in 25,000 years, the 
duration of the Tertiary Period down to the commencement of the 
Niveal Period, or Age of Snow, may have been about 93,000,000 
years. 
XI. Tue Surrace-Revier or CumBERLAND.—(qa) It has often been 
remarked that a right understanding of the various stages by which the 
surface-relief of any district has been reached involves reference to the 
whole of the later geological changes which that district has undergone. 
It also requires, especially, that we should know much concerning the 
developmental history of its rivers. This will be found to be true of 
Cumberland more perhaps than of any other county in the kingdom. 
The subject is, therefore, one that would need considerable space 
for its full consideration ; but it may be possible to give a general idea 
of the essential points even within the limits of a short article like the 
present. 
(4) The first principle to be borne in mind is that rivers of all 
kinds and of all countries have themselves shaped the valleys in which 
they flow. That is to say, no valleys, whether in Cumberland or else- 
where, are due to the mechanical severance of the rocks of which their 
valley-sides consist. Nor are they due to violent or sudden action of 
any kind soever. None of them, again, are the work of the sea. They 
are, one and all, simply depressions produced by the quiet and slow 
removal of rock-material by the gentle and prolonged action of rain and 
rivers. The chemical action set up by the acids in surface waters— 
especially by the humus acids—and by the oxygen in the atmosphere, 
have been amongst the most potent agents concerned ; and they have 
had, as auxiliaries in the work, the effects of heat and cold, the 
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