The theory of the earth's constitution put forward in 
this volume is that the surface on which we stand belongs 
to a crust some thirty miles thick, floating on a substratum 
of slightly greater density. Below this substratum may 
be a solid nucleus, 7zzs¢ be if Sir W. Thomson's proof of 
the earth’s rigidity be accepted; but this book does not pro- 
fess to go deeper. The floating crust cannot be supposed 
to pos:ess much strength, so that the weight of mountain 
chains would break through it, unless they have beneath 
them Corresponding protuberances on the under side of 
the crust, which shall support them by the additional 
buoyancy so produced. A plastic crust under compres- 
sion would yield above and below, and thicken, as two 
pieces of hot sealing-wax spread out when pressed 
together, so as to give rise to such double bulges as are 
sipposed. However, it is shown that even on this theory 
contraction cannot have produced the whole of existing 
terrestrial inequalities of surface, and could hardly even 
have lifted the continents above the sea-level. The 
general result of these suppositions would be that the 
crust beneath the ocean basins must be denser and 
thinner than that beneath the continents. To every ocean 
depression must correspond a similar larger concavity 
‘below, and continental elevations must have much greater 
protuberances answering to them on the under-side. 
Thus, could we strip off the crust like a hide, and turn it 
over, we should find the under-side reproducing the upper- 
side, only with every feature magnified. 
This conception may be deemed at first sight strange 
and wild, yet it certainly affords an easy explanation of 
one or two rather singular phenomena. It was found 
during the Indian Survey that the mountain mass of the 
Himalayahs attracted a plummet much less than it ought 
. to do, and that the cavities which contain the waters of 
the ocean, instead of causing a diminution of attraction, 
show an increase. Now it is shown that the protuberances 
of light material below the former and the concavities filled 
with the denser substratum below the latter would pro- 
duce exactly such results. Also the hemisphere of water, 
which maintains its position in spite of continental attrac. 
tion, is thus sufficiently accounted for. Again, since the 
floating crust must sink wherever weighted, and rise 
wherever material has been removed, we see how vast 
thicknesses of sediment might be accumulated without 
much perceptible change of depth, and mountains suffer 
continual degradation, and yet never be entirely effaced. 
Another remarkable argument is derived from the 
observations in the St. Gothard Tunnel, which show that 
the rate of increase of internal temperature is slower there 
than beneath plain countries, and slowest where the 
mountain is highest. This should not be the case, per- 
ceptibly were the earth cooling as a uniform solid. 
Assu ning these rates to be uniform, and allowing for the 
cold due to the elevation, it is easy to calculate the 
depths at which aay particular temperature would be 
reachel. If there be a molten nucleus its surface should 
be a surface of uniform temperature. But the depth at 
which a temperature of fusion can be reached will be 
found far greater under the mountains than under the 
plains. Hence, says Mr. Fisher, the solid crust must 
have protuberances below, answering to the mountains 
above. 
This argument is weighty. It approaches near to 
NATURE — 
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[Ma 
demonstration. If this slower rate of temperature-increase 
below mountains were satisfactorily made out, and if we 
could be sure that the rate remained uniform at all 
depths, the existence of such protuberances would be 
almost proved. It is difficult to see what other supposi- 
tion could be made. 
crepancies at present experienced in the observations of 
such temperatures, and with the evidence that exists for 
a rate depending on the depth, asceptic is not quite com- 
pelled to assent. 
No theory of the earth’s crust can be complete which 
does not provide the machinery for earthquakes and 
volcanoes. Mr. Fisher, for this purpose, supposes his 
subterranean fluid to contain, in intimate union with itself, 
vapour in considerable quantities. 
retained in the fluid by the superincumbent pressure, as 
gas is in the liquid of a soda-water bottle, and will, if such 
pressure be removed, be disengaged from the molten 
matter as the gas disengages itself when the cork is 
drawn, though much more slowly, by reason of the vis- 
cosity of the fluid. This agrees with the view taken by — 
Prof. Judd in his recent volume on Volcanoes. It will be 
a novel idea for many of us to imagine the earth like a 
globular bottle of effervescert liquid, and its crust like ice 
covering a lake of aérated-water. But such a constitution 
would account for many of the phenomena of eruptions. 
The earthquakes which usually herald them, the rise of 
molten material in a fissure, the existence of permanently 
liquid lava like that in Kilauea, the quiescence of neigh- 
bouring vents, the growth, death, and revival of a volcano, 
all follow as natural consequences. The difference in the 
lavas ejected from adjacent craters and the supposed order 
of succession in the products erupted are also accounted 
for, but not so satisfactorily. The theory is a very im- 
portant one, and appears on the whole the most satis- 
factory that has yet been propounded. 
It is natural to suppose that the emission of the vapour 
from this substratum would tend to produce a contraction 
of the nucleus. When we consider how far the volume 
of the ocean exceeds that of the continents it is surprising 
to be told inthe chapter on the Extravasation of Water 
that this supposition cannot account for them. However 
it will be found on examination that much depends on 
the hypothesis. The supposition made is not local emis- 
sions of liquid producing cavities, but a general exudation 
and consequent crumpling of the crust. The analogy is 
not to the subsidences in Cheshire, where brine has been 
removed, but to the wrinkled skin of an apple as it dries. 
The reader of this volume must bear in mind that most 
of the numerical results from time to time obtained and 
used are deductions from assumed data, and not inde- 
pendent truths. Such is a statement which often occurs 
in the calculations, that the contraction required to pro- 
duce the existing inequalities of the earth’s surface is 
00105. He must also distinguish real confirmations of 
the theory such as the deviations in the plumb-line and 
the slower subterranean temperature-increase in the 
neighbourhood of mountains, from mere appearances of 
coincidence in numerical results. The latter are in several 
cases necessary consequences of identical assumptions. 
The agency of intruded dykes in producing elevation and 
compression does not seem altogether a natural one. We 
may conceive the crust passing down into fluid, but not 
rch 9, 1882 
However, with the wide dis- — 
This vapour is to be — 
. 
‘ 
