GEO 
the strata which are lowest in the plains 
being highest in the mountains. The strata 
of these mountains are often intermixed 
with veins of metal, running in all possible 
directions, and occupying vacuities which 
appear to be of somewhat later date than 
the original formation of the mountains. 
The volcanic mountains interrupt those 
of every other description, without any 
regularity, as if their origin were totally 
independent of all the rest. 
The internal constitution of the earth is 
little known from actual observation, for 
the depths to which we have penetrated 
are comparatively very inconsiderable, the 
deepest mine scarcely descending half a 
mile perpendicularly. It appears that the 
strata are more commonly in a direction 
nearly horizontal than in any other; and 
their thickness is usually almost equable 
for some little distance ; but they are not 
disposed in the order of their specific gra- 
vity, and the opinion of their following 
each other in a similar series, throughout 
the greater part of the globe, appears to 
rest on very slight foundations. 
From observations on the attraction of 
the mountain Schehallien, Dr. Maskelyne 
inferred the actual mean density of the 
earth to be to that of water as four and a 
half to one, judging from the probable den- 
sity of the internal substance of the moun- 
tain, which he supposed to be a solid rock. 
Mr. Cavendish has concluded more direct- 
ly, from experiments on a mass of lead, 
that the mean density of the earth is to 
that of water as five and a half to one. 
Mr. Cavendish’s experiments, which were 
performed with the apparatus invented 
and procured by the late Mr. Michell, ap- 
pear to have been conducted with all pos- 
sible accuracy, and must undoubtedly be 
preferred to conclusions drawn from the 
attraction of a mountain, of which the in- 
ternal parts are perfectly unknown to us, 
except by conjectures founded on its ex- 
ternal appearance. Supposing both series 
of experiments and calculations free from 
error, it will only follow that the internal 
parts of Schehallien are denser, and per- 
haps more metallic, than was before ima- 
gined. The density assigned by Mr. 
Cavendish is not at all greater than might 
be conjectured from observations on the 
vibration of pendulums; Newton had long 
ago advanced it as a probable supposition, 
that the .mean density of the earth might 
be about five or six times as great as that 
of water, and the perfect agreement of the 
GEO 
result of many modern experiments with 
this conjecture affords us a new proof, in 
addition to many others, of the accuracy 
and penetration of that illustrious philoso- 
pher. See Globes. 
GEOLOGY has for its object the struc- 
ture and formation of this globe : it, of 
course, embraces the consideration of the 
materials of which it is composed, and the 
circumstances peculiar to its original forma- 
tion, as well as the ditferent states under 
which it has existed, and the various changes 
which it has undergone. 
It necessarily follows, from the very li- 
mited depth within which our actual exa- 
minations have been made, that our facts 
and real observations are confined to what 
may be considered, comparatively, as 
merely the crust of the globe. With re- 
spect to its more internal part, we have hi- 
therto only been aided by conjecture, 
which, it must be admitted, has too fre- 
quently led to theories the most extrava- 
gant and absurd. From the experiments 
of several learned men, it, however, ap- 
pears, that the density of the globe is great- 
est towards its centre. Boscovich is of 
opinion, from his very ingenious calcula- 
tions, that the centre is a spherical nucleus, 
possessing an equal degree of density to 
within some leagues of the earth’s surface ; 
but although it is thus concluded, that the 
interior of the earth is solid, contrary to 
the conjectures bf several ancient philoso- 
phers, yet it is by no means pretended, 
that even in this its more solid parts, there 
may not exist cavities of a greater or lesr 
size, connected, perhaps, with each other, 
and extending considerably, in all probabi- 
lity, towards the surface. 
The solid masses of the globe, which have 
come within our examination, have been 
distinguished into primitive and secondary ; 
among the former, were placed the rocks 
of granite, gneiss, porphyry, serpentine, 
and limestone, of a peculiar character; and, 
among the latter, were considered the 
rocks of secondary lime stone, of phosphate 
of lime, of gypsum, and of some of the sand- 
stones ; of chalk, and of silex. This divi- 
sion is not, however, at present universally 
adopted ; other divisions having been as- 
sumed, which have appeared to agree bet- 
ter with the different systems which have 
been proposed : these divisions we shall 
therefore more fully notice, after pointing 
out the peculiarities of these several systems. 
The water is supposed, at present, to cover 
about three-fifths of the whole earth ; but 
