AMOUNT OF SUBAERIAL DENUDATION. 
113 
limestones, 3a, and schists, 3Z>, presenting numerous folds, 
and becoming more and more metamorphic and crystalline, 
until at length, although very different in age and strike, 
they much resemble in appearance the group No. 1. It is 
very seldom that in the same country one continuous forma¬ 
tion, such as the Silurian, is, as in this case, more fossiliferous 
and less altered by volcanic heat in its older than in its 
newer strata, and still more rare to find an underlying and 
unconformable group like the Cambrian retaining its orig¬ 
inal condition of a conglomerate and sandstone more per¬ 
fectly than the overlying formation. Here also we may re¬ 
mark in regard to the origin of these Cambrian rocks that 
they were evidently produced at the expense of the underly¬ 
ing Laurentian, for the rounded pebbles occurring in them 
are identical in composition and texture with that crystal¬ 
line gneiss which constitutes the contorted beds of the in¬ 
ferior formation No. 1. When the reader has studied the 
chapter on metamorphism, and has become aware how much 
modification by heat, pressure, and chemical action is re¬ 
quired before the conversion of sedimentary into crystalline 
strata can be brought about, he will appreciate the insight 
which we thus gain into the date of the changes which had 
already been effected in the Laurentian rocks long before 
the Cambrian pebbles of quartz and gneiss were derived 
from them. The Laurentian is estimated by Sir William 
Logan to amount in Canada to 30,000 feet in thickness. As 
to the Cambrian, it is supposed by Sir Roderick Murchison 
that the fragment left in Sutherlandshire is about 3500 feet 
thick, and in Wales and the borders of Shropshire this for¬ 
mation may equal 10,000 feet, while the Silurian strata No. 
3, difficult as it may be to measure them in their various 
foldings to the eastward, where they have been invaded by 
intrusive masses of granite, are supposed many times to sur¬ 
pass the Cambrian in volume and density. 
But although we are dealing here with stratified rocks, 
each of which would be several miles in thickness, if they 
were fully represented, the whole of them do not attain the 
elevation of a single mile above the level of the sea. 
Computation of the Average annual Amount of Suhaerial 
Denudation. —The geology of the district above alluded to 
may assist our imagination in conceiving the extent to 
which groups of ancient rocks, each of which may in their 
turn have formed continents and oceanic basins, have been 
disturbed, folded, and denuded even in the course of a few 
out of many of those geological periods to which our im¬ 
perfect records relate. It is not easy for us to overestimate 
