Vermont phÅ“nix. [volume] (Brattleboro, Vt.) 1834-1955, 1880-10-22
MATTER.
or the Rocks that Compose the Earth's
Crust.

P'ooVu thiouBu, COuslstiuB Cnleuy OF Bhnu
gravel and clay generally crowned with
superstratum of soil composed of disintegrat-
ed rock and organic matter The total thick
ness of these layers varies from a few inches
to one or two hundred feet, or more, the val
leys, and locations favorably situated to re-
tain deposits-Rs the mouths OF riyers-beng
covered to greater depth, while mountain
Gus regions have thinner covering chiefly
through the agency of water and the tenden
cy Of every material body to lower position.
The thickness of the enrth's crust has been
estimated at fifty miles, the estimate being
based upon an increase of temperature at the
rate of L for every "a feet, upon a general
average, after descending 20 feet, which, at
that depth, would be sufficiently intense to
liquefy every known rock
1t may be observed here that the word
"rock, " in the language of geology, is very
comprehensive term, embracing not only
those substances that in common language
are called stone, as granite, gneiss and quartz I;
it also embraces sand, gravel and clay when
they form portion OF the geological series,
and the metallic ores Indeed, it compre.
hends all natural solids not embraced under
the term "organic matter, " and some of these,
as peat, coal, etc
For greater convenience, geologsts have
divided the rocks into three great classes, not
arbitrarily, but in accordance with nature, in
dicating their manner of formation, viz: ig-
neous, sedimentary and metamorphic rocks
The igneous rocks, ~ the word indicates, have
derived their character from tho ooapcv ~
fire deep in the bowels of the earth, and are
subdiyided into two groups, the volcanic aud
the plutonic, the former being the product Of
volcanic eruption, and generally porous and
cellular; the latter are more compact, having
cooled under the immense pressure of super.
incumbent matter. Granite, porphyry, sye-
mite and serpentine are plutonic rocks The
sedimentary rocks have been formed by the
agency Of water. and are subdivded into
three classes-m mechanical, chemical and or
ganic rocks The mechanical rocks are form
ed OF the triturated particles of pre-e1isting
rocks by aqueous agency briefly described in
apreyious article. They include sand, gravel,
clay, or these substances consolidated into
conglomerate, stratifed rock The chemical
rocks have been precipitated; these include
rock salt, iron ore, gypsum, and others. The
organic rocks are indebted to the agency Of
organic beings, animal or vegetable, for their
peculiar Characterization These include lime
stones coal, peat, shell mari, etc Metamor-
phic rocks are sedimentary, having been
formed by the agency of water; but, lying
immediately above the plutonic rocks. they
have mead al, l..
and have sometimes assumed crystalline tex
ture through several hundred feet The white
statuary marble is fine example of meta
morphic rock converted from the dark -colored
limestone by heat, sometimes beautifully mac.
ulated with metallic oxide. Among the white
marbles highly priced by the old Greeks were
the Pentelic and the Parian. The former was
obtained from Mount Pentelicus, the latter
from the island Of Paros The Parian was
regarded as the most beautiful, and it was in
this that those old masters, Scarpa, Praxi,eles
and Phidias wrought out their finest ideals,
that none in modern times have been able to
excel or scarcely to initiate.
The study of the rocks is made possible to
any large extent by the perpendicular motion
of the earth's crust Man can penetrate into
the earth but few hundred feet ut most, and
then his labor and his knowledge must end.
But upheayals Of the surface into mountain
chains, by which the rocks are broken up and
thrown into Wow, give opportunity to arrange
and classify them also, with an approach to
accuracy, 70 decide the question Of relative
age, and TO determine the thickness of differ
ent systems by measurements upon their up
turned edges The plutonic rocks, which lie
below all others. are thrown up into view
through rents along the anticlinal axes of
mountain ranges, and are thus accessible to
man, and contribute to science and the works
of art Hence granite should constitute the
axes of tho principal mountains upon the
globe and it is true that this valuable rock is
not found except in mountainous districts,
and not here unless there has been an upheav-
al sufficient to throw of the superintendent
rocks.
1t would be interesting, if not tedious, to
give the thickness of the several formations
in the British Isles, as recorded by an emf
nent Scotch geologist and published in 1871.
The Cretaceous system is estimated at GOO
feet, the New Red Sandstone at 500 feet, the
Carboniferous system at 300 feet, the Lower
Silurian at 300 feet, ETC., etc.
Involved in the study of the rocks are two
interesting questions-their actual and their
relative age The former, through lack OF
reliable data, presents far greater difficulties
than the latter. In different countries under
similar parallels of latitude, the geologist is
able to decide which strata were deposited in
the same period by their fossils. II system
in England and one in North America are
found to contain similar organic remains, it is
hence inferred that they were deposited in
the same period for, in remote periods, ami,
mails and plants, though less humorous than
at present, were widely distributed 1t is ev.
| ident that the lowest sedimentary rocks were
deposited first hence the relative age of the
rocks Of the same country is accurately deter
mined by the order of superposition.
The question of actual age must ever be
largely conjectural. 1t has been estimated
that stratum two inches in thickness is form
ed in a century. If this is correct in certain
cases, it is evident that it cannot be in all
Taking this as the average, together with DR.
Buckingham's estimate of the thickness oi
European strata at ten miles as the data, i
required 81, $50, 600 years for their formation
To carry our speculation still further (for i
must be admitted that this calculation is no
strictly demonstrable), II TO these figures b'
added the immense period that has elapsed
since these rocks were deposited, by record
ing their elevation at long intervals (see No
y, we have a realization of the great antiqui
ty of our planet, without inquiring the age o
the plutonc rocks. Moreover, there are nj
means of determining how many times tn,
rocks have been reduced and reformed unde
the destructive and conservative agencies al
ways in activity but it is not improbable
that such alterations may have taken plac
many times. If this be true, as argued b.
Tnnlmin and othera the atupendous age o