
*, 
- 
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. 
"Parr I. Secr. iii] UPHEAVAL AND DEPRESSION. 287 
& thetically stated by Mr. Darwin, we may conceive that after the outer 
parts of the globe had attained a considerable rigidity and could 
then be only slightly influenced by internal distortion, the effects of 
continued secular contraction would be seen in the intermittent 
; subsidence of the oceanic basins already existing, and in the 
successive crumpling and elevation of the intervening stiffened 
terrestrial ridges. 
This view, variously modified, has been widely accepted by 
geologists as furnishing an explanation of the origin of the upheavals 
- and subsidences of which the earth’s crust contains such a long record. 
But it is not unattended with objections. The difficulty of con- 
ceiving that a globe possessing on the whole a rigidity equal to that 
of glass or steel could be corrugated as the crust of the earth has 
been, has led some writers to adopt the hypothesis already described 
(ante, p. 53), of an intermediate viscous layer between the solid 
- _erust and the solid nucleus, while others have suggested that the 
observed subsidence may have been caused, or at least aggravated, 
by the escape of vapours from volcanic orifices. But with modifica- 
tions the main cause of terrestrial movements is still sought in 
secular contraction. 
Some observers, following an original suggestion of Babbage,! have 
supposed that upheaval and subsidence, together with the solidification, 
erystallisation, and metamorphism of the layers of the earth’s crust, 
may have been in Jarge measure due to the deposition and removal 
of mineral matter on the surface. There can be no doubt that the 
lines of equal imternal temperature (isogeothermal lines) for a 
considerable depth downward, follow approximately the contours of 
the surface, curving up and down as the surface rises into mountains 
or sinks into plains. ‘The deposition of a thousand feet of rock will, 
of course, cause a corresponding rise in the isogeotherms, and if 
we assume the average rise of temperature to be 1° Fahr. for every 
50 feet, then the temperature of the crust immediately below this 
_ deposited mass of rock will be raised 20°. But masses of sediment 

of much greater thickness have been laid down, and we may admit 
that a much greater increase of temperature than 20° has been 
effected by this means. On the other hand, the denudation of the 
land must lead to a depression of the isogeotherms, and a con- 
sequent cooling of the upper layers of the crust. 
It may be conceded that in so far as the internal structure of 
rocks may be modified by such progressive increase of temperature 
as would arise from superficial deposit, this cause of change must 
haye a place in geological dynamics. But it has been urged that 
besides this effect, the removal of rock by denudation from one area 
and its accumulation upon another affects the equilibrium of the crust ; 
that the portions where denudation is active, being relieved of weight, 
rise, while those where deposition is prolonged, being on the contrary 
loaded, sink. This hypothesis has recently been strongly advocated 
} Journ. Geol. Soc. iii. (1834) p. 206. 
