life is dawn on the earth 18
"It appears that great beds of the Laurentian limestones are
composed of the ruins of the Eozoon. These rocks, which are white,
crystalline, and mingled with pale green serpentine, are similar in
aspect to many of the so-called primary limestones of other regions.
In most cases the limestones are non-magnesian, but one of them
from Grenville was found to be dolomitic. The accompanying strata
often present finely crystallized pyroxene, hornblende, phlogopite,
apatite, and other minerals. These observations bring the formation
of silicious minerals face to face with life, and show that their
generation was not incompatible with the contemporaneous existence
and the preservation of organic forms. They confirm, moreover, the
view which I some years since put forward, that these silicated
minerals have been formed, not by subsequent metamorphism in
deeply buried sediments, but by reactions going on at the earth's
surface.[Z] In support of this view, I have elsewhere referred to
the deposition of silicates of lime, magnesia, and iron from natural
waters, to the great beds of sepiolite in the unaltered Tertiary
strata of Europe; to the contemporaneous formation of neolite (an
aluimino-magnesian silicate related to loganite and chlorite in
composition); and to glauconite, which occurs not only in Secondary,
Tertiary, and Recent deposits, but also, as I have shown, in Lower
Silurian strata.[AA] This hydrous silicate of protoxide of iron
and potash, which sometimes includes a considerable proportion of
alumina in its composition, has been observed by Ehrenberg, Mantell,
and Bailey, associated with organic forms in a manner which seems
identical with that in which pyroxene, serpentine, and loganite
occur with the Eozoon in the Laurentian limestones. According to the
first of these observers, the grains of green-sand, or glauconite,
from the Tertiary limestone of Alabama, are casts of the interior
of Polythalamia, the glauconite having filled them by 'a species of
natural injection, which is often so perfect that not only the large
and coarse cells, but also the very finest canals of the cell-walls
and all their connecting tubes, are thus petrified and separately
exhibited.' Bailey confirmed these observations, and extended them.
He found in various Cretaceous and Tertiary limestones of the United
States, casts in glauconite, not only of _Foraminifera_, but of
spines of _Echinus_, and of the cavities of corals. Besides, there
were numerous red, green, and white casts of minute anastomosing
tubuli, which, according to Bailey, resemble the casts of the holes
made by burrowing sponges (_Cliona_) and worms. These forms are seen
after the dissolving of the carbonate of lime by a dilute acid.
He found, moreover, similar casts of _Foraminifera_, of minute
mollusks, and of branching tubuli, in mud obtained from soundings in
the Gulf Stream, and concluded that the deposition of glauconite is
still going on in the depths of the sea.[AB] Pourtales has followed
up these investigations on the recent formation of glauconite in
the Gulf Stream waters. He has observed its deposition also in
the cavities of _Millepores_, and in the canals in the shells
of _Balanus_. According to him, the glauconite grains formed in
_Foraminifera_ lose after a time their calcareous envelopes, and
finally become 'conglomerated into small black pebbles,' sections
of which still show under a microscope the characteristic spiral
arrangement of the cells.[AC]
[Footnote Z: _Silliman's Journal_ [2], xxix., p. 284; xxxii., p. 286.
_Geology of Canada_, p. 577.]
[Footnote AA: _Silliman's Journal_ [2], xxxiii., p. 277. _Geology of
Canada_, p. 487.]
[Footnote AB: _Silliman's Journal_ [2], xxii., p. 280.]
[Footnote AC: _Report of United States Coast-Survey_, 1858, p. 248.]
"It appears probable from these observations that glauconite is
formed by chemical reactions in the ooze at the bottom of the sea,
where dissolved silica comes in contact with iron oxide rendered
soluble by organic matter; the resulting silicate deposits itself in
the cavities of shells and other vacant spaces. A process analogous
to this in its results, has filled the chambers and canals of the
Laurentian _Foraminifera_ with other silicates; from the comparative
rarity of mechanical impurities in these silicates, however, it would
appear that they were deposited in clear water. Alumina and oxide of
iron enter into the composition of loganite as well as of glauconite;
but in the other replacing minerals, pyroxene and serpentine, we
have only silicates of lime and magnesia, which were probably formed
by the direct action of alkaline silicates, either dissolved in
surface-waters, or in those of submarine springs, upon the calcareous
and magnesian salts of the sea-water."
[As stated in the text, the canals of Eozoon are sometimes filled
with dolomite, or in part with serpentine and in part with dolomite.]
(B.) Silurian Limestones holding Fossils infiltrated with Hydrous
Silicate.
Since my attention has been directed to this subject, many
illustrations have come under my notice of Silurian limestones in
which the pores of fossils are infiltrated with hydrous silicates
akin to glauconite and serpentine. A limestone of this kind,
collected by Mr. Robb, at Pole Hill, in New Brunswick, afforded not
only beautiful specimens of portions of Crinoids preserved in this
way, but a sufficient quantity of the material was collected for an
exact analysis, a note on which was published in the Proceedings of
the Royal Irish Academy, 1871.
The limestone of Pole Hill is composed almost wholly of organic
fragments, cemented by crystalline carbonate of lime, and traversed
by slender veins of the same mineral. Among the fragments may be
recognised under the microscope portions of Trilobites, and of
brachiopod and gastropod shells, and numerous joints and plates
of Crinoids. The latter are remarkable for the manner in which
their reticulated structure, which is similar to that of modern
Crinoids, has been injected with a silicious substance, which is
seen distinctly in slices, and still more plainly in decalcified
specimens. This filling is precisely similar in appearance to the
serpentine filling the canals of Eozoon, the only apparent difference
being in the forms of the cells and tubes of the Crinoids, as
compared with those of the Laurentian fossil; the same silicious
substance also occupies the cavities of some of the small shells,
and occurs in mere amorphous pieces, apparently filling interstices.
From its mode of occurrence, I have not the slightest doubt that
it occupied the cavities of the crinoidal fragments while still
recent, and before they had been cemented together by the calcareous
paste. This silicious filling is therefore similar on the one hand
to that effected by the ancient serpentine of the Laurentian, and
on the other to that which results from the depositions of modern
glauconite. The analysis of Dr. Hunt, which I give below, fully
confirms these analogies.
I may add that I have examined under the microscope portions of the
substance prepared by Dr. Hunt for analysis, and find it to retain
its form, showing that it is the actual filling of the cavities. I
have also examined the small amount of insoluble silica remaining
after his treatment with acid and alkaline solvents, and find it to
consist of angular and rounded grains of quartzose sand.
The following are Dr. Hunt's notes:--
"The fossiliferous limestone from Pole Hill, New Brunswick, probably
of Upper Silurian age, is light gray and coarsely granular. When
treated with dilute hydrochloric acid, it leaves a residue of 5·9 per
cent., and the solution gives 1·8 per cent. of alumina and oxide of
iron, and magnesia equal to 1·35 of carbonate--the remainder being
carbonate of lime. The insoluble matter separated by dilute acid,
after washing by decantation from a small amount of fine flocculent
matter, consists, apart from an admixture of quartz grains, entirely
of casts and moulded forms of a peculiar silicate, which Dr. Dawson
has observed in decalcified specimens filling the pores of crinoidal
stems; and which when separated by an acid, resembles closely under
the microscope the coralloidal forms of arragonite known as _flos
ferri_, the surfaces being somewhat rugose and glistening with
crystalline faces. This silicate is sub-translucent, and of a pale
green colour, but immediately becomes of a light reddish brown when
heated to redness in the air, and gives off water when heated in a
tube, without however, changing its form. It is partially decomposed
by strong hydrochloric acid, yielding a considerable amount of
protosalt of iron. Strong hot sulphuric acid readily and completely
decomposes it, showing it to be a silicate of alumina and ferrous
oxide, with some magnesia and alkalies, but with no trace of lime.
The separated silica, which remains after the action of the acid,
is readily dissolved by a dilute solution of soda, leaving behind
nothing but angular and partially rounded grains of sand, chiefly
of colourless vitreous quartz. An analysis effected in the way just
described on 1·187 grammes gave the following results, which give, by
calculation, the centesimal composition of the mineral:--
댓글 없음:
댓글 쓰기