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lI'XE flECORDS OF THEl ROOKS.
No. VIII.
ia TERTIARY DBDS Or IIOBASIT TOWf,
'Although these palors profess to deal ex
oluively with the different strata soon in
and close to this city, I find it, nevertheless,
necessary to my purpose to stray a few miles
beyond the originally prescribed limits, as far
as the district of Seroll, where a very re
markable example of these shell deposits no.
cures.
At Midway Point, a promontory well
known in connection with the Sorell Cause.
way, a fossil oyster bed is to be scen occupy.
ing an elevation little short of one hundred
foot above the level of the son. The prevail.
ing rock of this district is a brown sandstone,
probably of carboniferous age, since it ap
lenrs to be an extension of the sandstone of
Hobart Town. This sandstone rises in bold
precipitous clilTh at the promon.
tory, and also on the western
shore of the estuary. In several places it is
much undermined by the waves, leaving over.
hanging ledges at the top of the cliffe. The
promontory is sparsely clad with vegetation
on account of the sterile character of the
alluvial soil. Through this soil the oyster
shells are interspersed, and so nu5Ierous are
they that for years back, I am informed, they
have been collected for burning into limO
Some of these shells are of immense size, the
impression of the adductor muscle, in some
instances, being as large as a crown piece.
Those exposed on the surfaes show signs of
considerable decay, the laminle separating
with a touch, while many of them exhibit a
remarkable net-line appearance through
having been perforated in all directions by
some slll.boring worms, probably the na1s.
On the Clarence side of the estuary, in ap
proaching the western extremity of the Sorell
Causeway, a low, sandy flat is traversed for
about two and at half miles and bearing a
dwarfed vegetation, principally consisting of
casuarina. This plateau hles been recently
reclaimed from the sea by natural agency,
and in making a road over it numberless
oyster-shells have been unearthed, and which
are in a similar condition to those on the
promlontory, and belong to the same species,
viz, ostrea edslis (edible oyster). The in
ference to be drawn from this fact of the
oyster-shell deposits is, that at a compara
tively geologically recent period,-say. the
pliocene, i. e., latter part of the tertiary
period-an oyster bed extended over the
whole area comprised within the limits of the
promontory and those of the plateau. Now,
if a line were drawn parallel to the horizon
fromii the highest part of this oyster-shell dI.
posit at Midway Point, it would be seen that
at the time the oysters lived the greater part
of the surrounding country was under water;
and we will fulrther observe
that the highest part of these
deposits corresponds in altitude with
the shell deposits on the banks of the
Derwent. But since oyster beds usually
exist at a considerable depth below'the surface
of the sea, at a mean of ten fathooms, while
the shell beds on the banks of the Derwent
p int to a littoral zone-i.e., coast lino-there
is little doubt that the fossil oyster beds at
Sorell are older than the Hobart Town 1shell
hods, vwhile at the same time both are of
Tertiary origin.
When the Hobart Town shell beds were
deposited two-thirds at least of the site of the
city were under water, the level of which
would about reach to the clock of St. David's
Cathedral; but when the oyster beds at
Sorell contained their living inhabitants, the
water at that time would have covered the
top of the Cathedral steeple. It will thus be
a5en that a very striking change between the
relative position of land and sea has taken
place during a very recent period in Tasma
nia.' These'Tertiary shell beds have their
representatives on the coasts of all the Aus
tralian colonies, showing that the rise of the
land in this part of the globe has been
uniform.
Of greater interest still to the pabeon.
tologist are the Tertiary fresh-water limestone
deposits, (tmavertin.) On the east bank of
the Derwent, at Lindisfarne, there is an
inlet named Geilston Bay. At a distance of
two or three hundred yards from the water's
edge a quarry has been opened in some beds
of travertin, which have been worked for is
period extending over thirty years. From
time to time some very beautiful fossils
have been brought to light. There are three
distinct beds of travertin, which are separa.
ted by strata of loam, sandy gravel, and stiff
brown clay. At one part of the Tertiary
period, probably the maiocene (middle
period), the spot occupied by travertin beds
was a large pool, or lagoon, which was sup
plied by streams highly charged with car
bonats of lime at intervals, and which being
chemically precipitated buried and preserved
whatever organisms were in the pool.
The greatest depth obtained in excavating
the limestone is little short of one huIndred
feet, and the strata exposed represent three
distinct periods of repose and subsequent
disturbance by heavy rains or floods
whentthe beds of gravel and lousin were laid
down. TThe fossils obtained in this quarry are
principally the shells of land snails f(Bulimus
asid Helix), with some supposed to be fresh
water (Planorbisi and Lymlelus),
the stems and leaves of plants,
and the bones of animals. 'With
regard to the latter, I have in my col
lection some bones which I dug out at a
depth of eighty feet, or thereabouts, from the
surface, and beneath two compact beds of
travertin, each having a mean thickness of
eight feet. I am unable to say whether they
belong to recent animals on account of their
fragmentary condition-a question that will
doubtless be shortly settied by one of the
first esteological authorities of the present
day.
By no means the least insteresting of the
fossils from this quarry are the vegetable
impressions, owing as a' rule to their beauti-
ful state of preservation, The most of the
speclmens hitherto obtained cannot with
any 'degree of certainty be asslgned
to existing speoles of flora,. There
is one leaf, however, frequently met with
that I cannot help regarding as being closely
allied to our modern dogwood (pounaderts
apaetea), and of which I lave some remark
able specimens covering a slab of travertin,
Even insects have been found in this lime
stone, and supposed to be a species of water.
beetle, while seed.vessels with all their
delicacy of structure are at times met with.
Between the quarry and the river there is a
dyke of porous basalt that has burst through
the travertin, dislocating and lifting up the
whole formation to a considerable height
above its original position. It was; in all
probability, this eruption of volcanic rock
that diverted the springs which supplied the
pool, whereby the deposition of travertin
ceased. At any rate, it serves to show that
volcanic action took plaee in Tasmania during
the Tertiary period and subsequent to the
laying down of this travortin.
On the western side of the Ilorwent, at the
upper end of Murray-street, there is another
travertin formation which was formerly quar.
rind for limo. It forms a bill of considerable
so'ivity having, without doubt, been caused
by tho eruption of trap, which in this case
completely ebatt rod the travertin strata into
smell masses which nro mingled htoero
geneously wiii, aiit were the alternating
beds. Here again we have another evidence
of voleanie nation during a recent geological
period. The travertin of this spot lies not
proved very fruitful in organic reomain.
Those fossils which have been brought to
light hear a striking resemblance to those ob.
tained in the first mentioned locality.
At Richmond there is a slid finer example
of Tertiary freshwater limestone, which is elso
worked for lime, and which, if anything. is
richer in foesils tiian the one at Goilston Bay.
One notable feature in this formation is the
siliceous tufa-in other worls concretions of
flint-which have been deposited by thermal
springs and which seem still to exist in a
modified form. This thermal agency would
seem to have played a conmplctons
part in the Geilston Bay deposits, for there
large masses and incrustations of flint are ex
posed. This, I may mention, is a most
interesting fact, and of no small importance
when viewed in connection with geological
inquiry in Tasmania.
I have now arrived at my journey's and.
I could go farther into the historio period
(post-plioecne) did I think I could direct
attention to formations and deposits having
sufficient interest for the general reader. My
object in these papers bas been simply to coil
attention to some of the more striking
mutations which this part of Tasmanis has
undergone, since the dawn of the Carboniferous
period up to the close of the Tertiary. In
looking over what I have written I find with
some sense of humiliation how much I have
fallen short of my original design. If I have 1
been fortunate enough, however, to awaken
ever so slight an interest in this most beautiful
an I ep.mnding branch of natural philosophy
by thosecrude and discesive remarks, so Ishall
feel fully compensated for the time bestowed i
on that which is ever to me a labor of love.
SI. 1.
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