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THE NATURE OF THE EARTH'S INTERIOR* 
[* a previous discourse on volcanoes,t I directed attention 
to the phenomena of volcanic action, specially considered 
in relation to the part which such igneous or internal forces 
have played in determining the grand features of the external 
configuration of the sphere upon which we live. 
If now we follow up this subject still further, it will naturally 
lead to an inquiry into the nature of the internal substance of the 
globe itself, within which the foci of such agencies must 
be situated ; quite independent of this, however, I have little 
doubt but that many of you must at some time or other have 
already asked yourselves the question—What does the central 
mass of the earth beneath us consist of ? 
The answer which, in the first instance, would, I imagine, be 
most likely to suggest itself to your minds, would be that it con- 
sisted of solid stone, such as you see forming the body of its 
mountains, the foundations of its continents, and the rock basins 
which contain itsseas. The belief in such an hypothesis would, 
however, be rudely shaken by the first personal experience of the 
shock of an earthquake, the sight of a volcano in eruption, or 
the consideration of the immense faults which have dislo- 
cated many parts of the solid land ; since, so far from disposing 
us to regard the ground under us as entitled to the appellation 
of the ¢erva firma, so commonly used by the ancients, the study 
of such phenomena could not but suggest grave doubts in our 
minds as to whether the earth was after all anything like so solid 
or stable as we at first sight felt inclined to imagine. 
But very little inquiry into the subject is necessary, however, 
to convince any one of the great difficulties in the way of obtaining 
a satisfactory answer to this question, and to prove that at 
present we do not have at our command sufficient data or evi- 
dence to enable us to arrive at a thoroughly conclusive solution 
of this most interesting problem. 
As the rapid advances made by the natural sciences are, how- 
ever, daily adding to our information on this subject, and thus 
enabling us to correct or modify our previous deductions, so as to 
form a more and more trustworthy opinion on the nature of those 
parts of our globe, -which, from their position, must always 
remain inaccessible to our powers of direct observation, I have 
imagined that a short sketch of the present state of our know- 
ledge concerning the probable constitution of the interior of the 
earth might prove interesting. 
In treating this subject, we will first consider what has already 
been done in the way of direct examination of the earth’s sub- 
stance in depth; yet when it is remembered that the mean 
diameter of our planet is some 7,912 miles, whilst the 
greatest depth hitherto attained by man’s direct exploration has 
not even yet reached one mile from the surface downwards ; this 
disproportion appears so enormous as to render it self-evident, 
in the pursuit of this inquiry, especially as regards the more 
central portions of the earth, that we must in the main rely upon 
data furnished by calling in the aid of the natural sciences. ‘The 
direct examination of the exterior of the earth, even when restricted 
to this depth, does nevertheless furnish us with many important 
data from which to start in this toa great degree speculative 
inquiry, and to some of these we shall now direct attention. 
It must first of all be remembered that all the rocks which 
we meet with at the surface, and which compose so much of the 
solid exterior of our globe as is actually known to us, may be 
arranged under two principal heads, viz., the volcanic or endo- 
genous rocks, 7.¢. those formed within the body of the earth 
itself, and the sedimentary or exogenous, 7.¢. those rocks formed, 
or rather reconstructed upon its surface, out of the déris of pre- 
viously existing rocks arranged in beds or strata by the mechani- 
cal action of water. 
It was long taken for granted by geologists that the lowest sedi- 
mentary strata, in their normal or in a more or lessaltered condition, 
rested directly upon granite, which was for a long time regarded as 
the foundation upon which they, in the first instance, had been de- 
posited, since this rock was looked upon as the oldest of all, and 
as representing the primeval or original surface covering of the 
earth, Later researches, however, have proved this hypothesis 
to be untenable, since it appears that no instance of a granite has 
as yet been met with in nature, which if followed up, does not at 
some place or other break through, and alter or disturb more or 
less, the stratified rocks in immediate contact with it, so that it natu- 
'* Substance of a Lecture delivered for the Sunday Lecture Society on 
January 29. 
+ See Nators; vol. ii p. 283. e 
NATURE 

| Feb. 9, 1871 
rally follows that such stratified rocks must have preéxisted on 
the spot, or in other words, that they were older in geological 
chronology than the granite which came to disturb them. 
In the present state of geology, however, it is utterly impos- 
sible for us to point out any variety of rock whatsoever as 
the one which may have served as a foundation upon wh ch the 
oldest sedimentary rocks were originally deposited, in fact the 
oldest rocks which we know of at present are themselves sedi- 
mentary rocks (mostly in an altered condition) belonging to the 
Laurentian series in Canada, and as yet it has not been found out 
what these sedimentary beds may in their turn rest upon, 7.¢., what 
is actually below them. es 
As therefore we have not as yet been able to reach down to 
investigate directly any rocks lower in the geological series than 
those pertaining to the Laurentian formation, we will now turn 
to the volcanoes, in order to examine the mineral products which 
they bring up for our consideration, from depths vastly beyond 
those which we can ever hope to reach directly. What vol- 
canoes teach us with regard to the nature of the earth’s interior, at 
a depth from which they derive their supply of mineral matter, 
may be summarised as follows :— 
That at this depth, the earth’s substance exists in a state of 
perfect molten liquidity, forming as it were a sea of molten rock, 
analogous in character to the eruptive rocks which have broken 
through the earth’s crust in former times. Secondly, that the 
mineral products ejected from volcanoes are very similar to one- 
another in chemical and mineral constitution, no maiter from 
what part of the globe they may emanate from. And, lastly, 
that from the same volcanic orifice, and during the same eruption, 
lavas of two totally different classes may be emitted, viz., the 
light, acid, or trachytic lava, analogous to the granites, felsites, 
&c., of the oldest period, and the heavy basic or pyroxenic 
lava, all but identical with the dark basaltic or trappean rocks 
commonly met with as dykes, &c., disturbing most of the 
different sedimentary formations. 
Another deduction from the study of volcanic phenomena, 
indicating that at a certain depth below their surface they must 
be in connection with a continuous sea of molten lava, is based 
upon the influence which the moon appears to have on 
volcanic eruptions ; this opinion seems to have been confirmed by 
the observations of Prof. Palmieri made during the last eruption 
of Vesuvius, on which occcasion he reported that distinct tidal 
phenomena could be recognised, thereby indicating that the 
moon’s attraction occasioned tides in the central zone of molten 
lava, in quite a similar manner as it causes them in the ocean. A 
further corroboration of this view is seen in the results of an ex- 
amination of the records of some 7,000 earthquake shocks which 
occurred during the first half of this century, compiled by Perry, 
and which, according to him, demonstrate that earthquakes are 
much more frequent in the conjunction and opposition of the 
moon than at other times, more so when the moon is near the 
earth-than when it is distant, and also more frequent in the hour 
of its passage through the meridian. 
Returning now to the more direct examination of the superficial 
parts of the earth, we find that the results of mining operations 
have also thrown considerable light, not only on the mineral 
nature of the rocks encountered in depth, but also upon some of 
their physical conditions. A numerous set of experiments made 
in deep mines in various parts of the world, often far distant 
from one another, has most conclusively proved that the tem- 
perature of the earth, at least as deep down from the surface as 
has been explored by man, increases in direct ratio as we descend 
towards its centre. Other observations on the temperature of the 
water from deep-seated and hot springs, and from artesian wells, 
fully confirm the experiments made in mines, and show that the 
temperature of the water furnished by them also becomes higher 
in proportion to the depth of the source from which it is derived. 
As might naturally be expected, the interference of local causes 
renders it a matter of considerable difficulty to determine the 
true mean general rate of such increase in temperature of the 
earth’s substance downwards ; still, in the main, observers all agree 
in placing it at somewhere between 1}° and 23° degrees F. 
for every hundred feet in depth, so that we cannot be far 
wrong, if for our purpose we estimate it at 2° F. for every 
hundred feet in depth, or a rate which amounts to 121° for each 
geographical mile nearer the earth’s centre. Since no facts are at 
the present time known which can in any way invalidate the 
supposition that this or a somewhat similar rate of increase in 
temperature holds good in still greater depths, it is perfectly 
correct and justifiable reasoning to assume that such is actually 

