a ; 
wt q 
Parr L] CONDITION OF THE EARTI’S INTERIOR. 53. 
_ presence in the interior of intensely heated metallic substances, we 
cannot suppose that solidified portions of such rocks as granite and 
the various lavas could ever have sunk into the centre of the earth, 
so as to build up there the honey-combed cavernous mass which 
might have served as a nucleusin the ultimate solidification of the 
whole planet; though the earliest formed portions of the compara- 
tively light crust would no doubt descend until they reached a 
stratum with specific gravity agreeing with their own, or until they 
were again melted.’ 
3. Hypothesis of a liquid substratum between a solid nucleus and the 
crust.—Since the early and natural belief in the liquidity of the 
earth’s interior has been so weightily opposed by physical arguments, 
geologists have endeavoured to modify it in such a way as, if possible, 
to satisfy the requirements of physics, while at the same time 
providing an adequate explanation of the corrugation of the earth’s 
crust, the phenomena of volcanoes, &c.’ Professors Shaler? and Le 
Conte,* and Mr. Fisher ® have advocated the existence of a fluid or 
viscous substratum beneath the crust, the contraction and consolida- 
tion of which produced the corrugations of the rocks and of the 
surface. “The increase of temperature,’ says Mr. Fisher, ‘ though 
rapid near the surface, becomes less and less as we descend, so that, 
if the earth were once wholly melted, the temperature near the 
centre is not very greatly above what it is at a depth which, compared 
to the earth’s radius, is small. Consequently, if it requires great 
pressure to solidify the materials at such a temperature, it is probable 
that the melting temperature may be reached before the pressure is 
‘sufficient to solidify.” The crust, of course, must be able to sustain 
itself on the corrugated surface of the supposed viscous layer without 
breaking up and sinking. ‘The same writer has suggested that 
_ the observed amount of corrugation is more than can be accounted for 
even on this hypothesis, and that the shrinkage may have been due 
not merely to cooling, but to the escape of water from the interior in 
_ the form of the super-heated steam of volcanic vents.’ More recently 
Herr Siemens has been led, from observations made in May 1878 at 
Vesuvius, to conclude that vast quantities of hydrogen gas, or com- 
_ bustible compounds of hydrogen, exist in the earth’s interior, and that 
these, rising and exploding in the funnels of volcanoes, give rise to 
the detonations and clouds of steam.’ 
It must be admitted that the wide-spread proofs of great 
erumpling of the rocks of the crust present a serious difficulty, for 
1 See D. Forbes, Geol. Mag. vol. iv. p. 435. 
2 See Dana in Silliman’s Journal, iii. (1847) p. 147. Amer. Journ. Science (1873). 
3 Proc. Bost. Nat. Hist. Soc. xi. (1868) p. 8. Geol. Mag. v. p. 511. 
4 Amer. Journ. Sct. 1872, 1873. 
5 Geol. Mag. v. (new series) pp. 291 and 551. See also Hill, op. cit. pp. 262, 479. The 
idea of a viscous layer between the solidifying central mass and the crust was present 
in- Hopkins’ mind. Brit. Assoc. 1848. Reports, p. 48. 
& Phil. Mag. Oct. 1875. 
7 Monatsbericht der K. preuss. Akad. Wéssenschaft, 1878, p. 558. See also Book: iii. 
_ Part i. for an account of Fouqué’s observations on the discharge of hydrogen at Santorin. 
