Few cities can boast of such a variety of beautiful scenery in its
immediate vicinity as occurs within a short distance from Dublin. We
need not allude to the objects of deep historical interest with which
the natural beauties of Dublin are associated, as they have often been
illustrated in these pages. The picturesque beauties of Dublin Bay and
the county of Wicklow are known to all; but it is less generally known
that the same localities abound in matters well calculated to excite
the curiosity of the naturalist. From the great variety of rocks, and
consequently of soil, around Dublin, we find a corresponding variety in
its vegetable productions; and we believe we are pretty correct when we
state that the botanist may collect specimens of nearly two-thirds of
the indigenous plants of Ireland within the distance of a few miles from
the capital. As regards Zoology, or the study of animals, our position
is equally fortunate. The shores near Malahide are uncommonly rich in
marine productions, especially shells; and the Bay of Dublin is not
inferior to the coasts of Devonshire for the variety of its zoophytes and
corallines. In the work of Ellis on British Corallines, we find that,
although that admirable naturalist resided in London, he obtained many
of his finest specimens from Dublin. In respect to mineralogical and
geological pursuits, we are equally well situated. At Killiney and in the
mines of Wicklow several interesting and some very rare minerals may be
collected. In geology, in the strict sense of the word, there are many
curious phenomena which should be repeatedly examined by the student, and
he will find such a mode of proceeding infinitely more profitable than
the more indolent method of confining his researches to such instruction
as can be found in books and sections. At Howth, or the promontory of
Bray, he may examine every diversity of stratification, and may observe
all the upheavings and contortions to which rocks have been exposed,
displayed as in a model, open to the contemplation of the man of science,
and to the instruction of all. The granite veins of Killiney are also
extremely curious, and well deserve to be repeatedly visited by the
beginner in geological pursuits. It is true that the questions to which
such phenomena gave rise have been long since set at rest; but it is also
true that the questions must be mastered by every student, and we know of
no place where this can be done to more advantage than at Killiney.
Every one is aware that rocks are formed in two very different ways, they
may be produced either from the decayed materials of older rocks, carried
down to the sea or lakes by the rivers, and subsequently consolidated
by various processes, which geologists have explained, or they may be
formed by the solidifying of liquid matter poured forth through some
volcanic aperture from the deeper parts of the earth. The first kind of
rocks are disposed in layers, beds, or strata, by the return of water,
and hence are called stratified, and also aqueous or water-formed; the
second, being liquid matters which have become hard from cooling, are
called igneous, or fire-produced rocks. As volcanoes are at present
confined to particular regions of the earth, some may imagine that such
igneous rocks should only be found in volcanic regions. This, however,
is a mistaken supposition, for geology assures us that igneous rocks are
to be found in every mountain range. The mode of reasoning which they
follow is equally simple and convincing. If we visit Howth, for example,
we find many of the strata resting on their edges, or variously twisted.
At the Killerys in the west of Ireland we find strata composed of rolled
pebbles, elevated to a very considerable angle. It is impossible that
strata of loose sand or gravel would have been originally deposited in
such inclined positions, and we know of no natural power which would
elevate them but that of the igneous agency, producing either a violent
earthquake, or a long-continued upward pressure. This opinion is much
strengthened, when we find in every country, whether volcanic or not, a
series of rocks which appear to have been violently inserted among the
strata, and which we can prove were once in a state of intense heat and
fusion, like the lavas from a modern volcano.
The granite of Killiney is one of those igneous rocks, and the
appearances which we detect in that interesting locality afford
satisfactory evidence of its mode of formation. When we descend to
the shore by the stairs, a little to the east of the Obelisk, we find
ourselves in a little way bounded by perpendicular rocks. These rocks are
of two kinds--granite, and a schistoze or slaty rock, of a bluish colour,
which we may term mica-schist. We then observe that the mica-schist
rests on its edges, on a pavement of granite, and also reclines against
that rock. The junction of the two rocks may be seen with the utmost
perspicuity; and there is no blending of their characters, even where
they are in absolute contact. We may next observe a ledge of rock
partly covered by the waves, and extending in nearly a north and south
course along the shore. This is a granite vein of many feet in breadth,
and several hundred yards in length, and may easily be traced for a
considerable distance. This granite vein is bounded on both sides by
mica-schist; and, what is still more important, we may follow the vein
till it is lost in the general mass of granite of the hill. When we now
remember that the water-formed rock (the mica-schist) is standing on
edge, a suspicion arises that the granite is a fire-produced rock, and
has been the agent of this elevation, and the large wall of granite may
have been intruded in a molten state between the beds of mica-schist. If
it be objected that the granite vein is merely a portion of the strata
of mica-schist, and was like them deposited from water, an inspection
will dissipate this illusion; for we observe that the great vein running
parallel to the strata gives off a smaller vein at right angles to the
direction of the strata. On examining this smaller vein, which may be
seen a little to the north of the stairs, all doubts respecting its
nature or origin are very soon removed. We are surprised to find that
this vein contains fragments of the mica-schist. We may therefore
conclude from this that originally fissures were produced in the schist,
and these fissures were filled up by molten granite, which entangled
fragments of the mica-schist which fell from the sides of the fissure. It
is scarcely necessary to add, that we know of no agent capable of melting
granite but heat.
When we examine this interesting spot a little more minutely, we detect
many other granite veins, each affording some curious and minute fact in
harmony with the preceding remarks. Every one knows that it is easier to
split a piece of wood in the direction of the grain, than transversely
to that direction. In the same way we may infer that it is easier for a
liquid granite to insinuate itself _between_ the strata than to force
its way _across_ them, and on examination we find this to have been the
case. In the first place, the large vein first mentioned running in
the course of the strata is broader than all the transverse veins put
together. Secondly, when we examine the cross veins, we find they have
had more difficulty in forcing their way: hence they frequently contain
fragments. Perhaps, however, an examination at another point near the
entrance of the abandoned lead-mine affords the most curious evidence
of these remarks, for there we perceive that the vein does not hold a
straight course, nor is it of equal thickness throughout, but, on the
contrary, is of unequal breadth, and serpentine, as if the strata had
been violently lacerated instead of being split. In this case the vein
has cut across the strata, and includes fragments of the mica-schist. But
the most curious circumstance in this example is, that the vein itself
has been broken, and its fractured extremities a little displaced and
detached, thus proving that the strata had been exposed to concussion and
displacement at a period posterior to that when the vein was formed.
If this very brief description will induce any of our readers to
visit the granite veins of Killiney, we are sure he will find that
his excursion will not be an unimproving one, and he will perhaps be
convinced that he has only to look about him to find sources of enjoyment
which so many are ignorant of, but which are within the command of all.