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THE MEDITERRANEAN NATURALIST 
however, take the low rats of denudation of 1 foe ., 
in 4.000 years. (:} Then, in 100,000,000 year, the' 
denudation would amount to 25,000 feet, distri- 
buted evenly over the land areas of the globe. In 
mountain regions this figure would be exceeded 
many fold, as calculations are given by American 
geologists of denudations equal to, or exceeding, 
this that have taken place in late geological per- 
iods in particular Pereas of the United States. 
My own opinion is that the denudation of the 
land areas of our globe since the dawn of geological 
history has vastly exceeded this figure, but as 
Mercutio in “Borneo and Juliet” says of his wound, 
“ ’Tis not so deep as a well, nor so wide as a church 
door ; but ’tis enough, ’twill serve.” 
In round figures, live miles of denudation would 
equal the maximum calculation of the de; th of the 
level-of-no-strain. What would be the effect of 
this conditioning agency on the position of the 
level-of -no-strain? In the absence of other anta- 
gonistic forces if would remove most of the 
evidences of compression produced by secular 
contraction in the outer iind of the earth. 
The problem is an exceedingly complex one, and 
to some extent involves the question of the per- 
manence of continents and oceans; but ever- 
granting for the sake of argument what I do not 
believe .is the fact, that continents have a ’ways 
been where they are now, the removal of this 
amount of rock would of itself lower the isogeo- 
therms to the same extent. That iy if a layer of 
the earth situated rive miles deep were denuded of 
its covering till it reached the surface, it would 
have its temperature lowered, contraction would 
take place, and the conditions upon which the 
calculations of the depth of the level-of-no-strain 
are made would be materially modified. On the 
other hand if, as I believe with Hutton, Lyell, and 
the older school of geologists, every portion of the 
surface of the globe has been at one time or another 
subject to denudation, and the total amount of 
material removed from one sits to another is much 
in excess of the calculation given, the non-existence 
of corrugations traceable to secular contraction 
is not remarkable. 
(1) See “ Denudation of the tivo Americas .” — 
Presidential Address, Liverpool Geological Socie- 
ty, 1884-5. 
But as I have pointed out elsewhere (1) if the 
shrinkage of the Earths nucleus were an active 
force now, the outer crust would be in a constant 
state of stress so that any artificial excavation 
made in rock in any part of the globe would re- 
lease a portion of this stress to the extent of the 
area and depth of such excavation. No such 
general state of stress in the Earths Crust is dis- 
coverable but local stresses do exist which are 
released by quarrying operations so that trenches 
cut across the floor of a quarry sometimes sud- 
denly close up, instances being detailed in the 
paper referred to. The rarity of this interesting 
phenomenon is a proof that the Crust of the 
Earth at the surface is not generally in a state of 
compression. 
It is impossible for me, within the limits of 
these papers, to go into details as io the way the 
folds of mountain ranges have been produced. I 
am compelled to refer the reader to my original 
work on the subject, and to the diagrams and 
examples therein. I may mention, however, that 
the lengths of the mountain folds measured along 
the courses of the beds and the reproduced denu- 
ded arehs are, fallaceous as a test of the original 
lengths of the beds. In many cases the length of a 
bed has been increased by lateral squeezing, and 
often the arch has never extended over the moun- 
tain tcp. It has been broken and separated by 
the protrusion of the underlying beds. 
But if expansion is produced by the rise of 
temperature induced by sedimentation, contraction 
will result from the lowering of temperature 
initiated by denudation. Do we find geological 
evidences of such contraction? will be the first 
question of the practical investigator. To which 
I answer yes; the phenomenon of normal faulting 
is due to contraction. 
It is strange, but true, that the theorists who 
refer the corrugation of the earth’s surface to 
secular contraction, pass over the difficulties pre- 
sented by the existence of normal faults very 
lightly. The explanation generally offered is that 
they are fractures of the earth’s crust produced by 
its sinking upon the cooling nucleus. This expla- 
nation is not very much more lucid than if we were 
( 1 ) The Cause of Active Compression stress in 
rocks and recent rock Flexures. American Jour, 
of Science. Vol. XLI. 1891 p.p. 409 -41 f. 
