1877] • Probable Origin and Age of the San. 31:3 
mile must have been denuded, but we do not know how 
much more than that may have been removed. For anything 
which we know to the contrary hundreds of feet of rock 
may have been removed before the dislocation took place, 
and as many more hundreds afterall indications of dislocation 
had been effaced at the surface. 
But it must be observed that the total quantity of rock 
which has been removed from the present surface of the land 
is evidently small in proportion to the total quantity re- 
moved during the past history of our globe. For those 
thousands and thousands of feet of rock which have been 
denuded were formed out of the waste of previously existing 
rocks, just as these had been formed out of the waste of 
yet older rock-masses. In short, as a general rule, the rocks 
of one epoch have been formed out of those of preceding 
periods, and go themselves to form those of subsequent 
epochs. 
In many of the cases of enormous denudation to which 
we have referred, the erosion has been effected during a 
limited geological epoch. We have, for example, seen that 
upwards of a mile in thickness of Carboniferous rock has 
been denuded in the area of the Pentlands. But the Pent- 
lands themselves, it can be proved, existed as hills, in much 
their present form, before the Carboniferous rocks were laid 
down over them ; and as they are of Lower Old Red Sand- 
stone age, and have been formed by denudation, they must 
consequently have been carved out of the solid rock between 
the period of the Old Red Sandstone and the beginning of 
the Carboniferous age. This affords us some conception of 
the immense lapse of time represented by the Middle and 
Upper Old Red Sandstone periods. 
Again, in the case of the great fault separating the Silu- 
rians of the south of Scotland from the Old Red Sandstone 
tradts lying to the north, a thickness of the latter strata of 
probably more than a mile, as we have seen, must have been 
removed from the ground to the south of the fault before 
the commencement of the Carboniferous period. And again, 
in the case of the Lancashire coal-fields, to which reference 
has been made, nearly two miles in thickness of strata had 
been removed in the interval which elapsed between the 
Millstone Grit and the Permian periods. 
As we are enabled, from geological evidence, to form some 
rough estimate of the extent to which the country in various 
places has been lowered by sub-aerial denudation during a 
given epoch, it is evident that we should have a means of 
arriving at some idea of the length of that epoch, did we 
