58 REPORT—1862. 
to that upward tilting, vast masses of once superincumbent beds having been removed 
from aboye them. ‘Ihese hills are high, not in consequence of, but in spite of de- 
nudation. I haye elsewhere proposed to call the first kind “ hells of curcumdenu- 
dation,” and the second “hills of uptilting.” To the latter class belong all the 
great mountain-chains of the world, and most of the smaller ones. 
It may be taken as an inyariable rule, that, as we approach all mountain-chains 
formed by uptilting, the beds rise towards them, and end successively at the sur- 
face; lower and lower beds still rising up, until the lowest of all appear in the 
heart of the mountains, where they are often reared up into the loftiest peaks. 
True as is this general statement, it is only generally true. The great groups of 
rocks thus rise successively one from beneath another; but this general rise is often 
complicated by numerous folds and reduplications, by great longitudinal fractures, 
or by complex flexures. 
The geological axis of a mountain-chain runs along the line where the lowest 
group of beds rises to the surface. The geogra, hical axis may be said to run along 
that dominant crest which forms the watershed of the chain. But it by no means 
follows that these two axes are coincident, that the lowest group of beds is always 
confined to the line of watershed, or eyen that the loftiest peaks and summits rise 
from that crest. The geological axes are dependent solely on the internal forces 
of elevation; if, therefore, the geographical axes do not coincide with them, it shows 
at once that they are independent of those forces; in other words, that the great 
external features have not been caused by the direct action of internal movement. 
The position of the geological axes of mountain-chains has, however, been often 
erroneously placed, from the tendency to refer them to any great masses of granite 
or other plutonic rocks that may show themselyes,—a reference which is more often 
erroneous than correct. 
All mountain-chains of uptilting tell the same story, that if the internal forces 
of disturbance and elevation had acted alone, without any external action of denu- 
dation, and if they had acted without it to the same extent which they haye with 
it (supposing that possible), the mountain-chains would haye been many times more 
lofty than they are. I say “supposing that possible,” because it appears to me that 
the elevation of the lowest rocks might never have proceeded to the same extent, 
if the internal force had not been gradually relieved of some of the external weight 
which it had to lift. However that may be, we see now that the lowest beds 
which appear at the surface, about the geological axis of a mountain-chain, dip 
on either hand beneath an ever-increasing thickness of superincumbent rock, as 
we recede from the axis. All the rocks which have been affected by the same 
action of disturbing force must have stretched unbroken across the disturbed district, 
before the disturbance commenced; for the lowest rocks appear at the surface 
now, not in consequence of the flexure or fracture of those that were aboye them, 
but in consequence of their removal, That remoyal could not haye taken place 
prior to the internal disturbance, unless we assume the existence of a deep hole or 
trough of erosion along the space where the mountain-chain was subsequently thrust 
upwards. The remoyal of the hent or broken beds, then, must have taken place 
either during the action of disturbance or subsequently to its termination. In 
either case it was an external action, the result, in fact, of moving water, which 
slowly wore away and carried off so many square miles or, as in some cases, so 
many hundreds or thousands of square miles of rock, so many thousands of feet in 
thickness. The internal forces operated simply by lifting up the rocks to within 
the region of the denuding influence, and they have only produced that indirect 
effect upon the features of the surface which results from their haying brought 
up to different levels, and placed in yarious positions, masses of rock of yarions 
hardness and constitution, on which the forces of erosion and transport have had a 
corresponding yariety of effect, when they reached them. 
I believe that all our uptilted mountain-chains haye thus grown by a very slow 
and gradual growth, the internal force thrusting upwards what the external agen- 
cies always tended to wear down. 
The investigation of the nature and effects of the mechanical forces that have 
acted on the crust of the earth from the interior has been undertaken by many 
eminent philosophers, by none with more acuteness and profundity than by our pre- 
