OF THE ROCKS CALLED BASALTIC.
In some parts of the world, there are rocks which are not stratified
and arranged like those which I described to you some time ago, but are
laid over the surfaces, and in the crevices of others in this manner:--
[Illustration]
In this cut, _a_, _a_, _a_, are intended to represent the stratified
rocks, and _b_, _b_, _b_, the others, which from their position are
sometimes called _overlying rocks_, and they mostly consist of a
substance called _basalt_, which is nearly black and very hard, and
occasionally with white and coloured minerals imbedded in it. There are
two states in which it is found; in one, it occurs in masses without
any particular form, and in the other state it constitutes pillars
shaped with great accuracy and regularity.
[Illustration: PLATE VIII.
STAFFA.]
The island of Staffa, one of the western islands of Scotland, and the
Giant's Causeway, on the north coast of Ireland, near Londonderry, are
remarkable instances of the latter state; and as I have seen them both,
I will tell you what sort of places they are.
STAFFA.
This wonderful island was not generally known to exist, even by the
inhabitants of Scotland, till the last century, when Sir Joseph Bankes
happened to see it, and published an account of it. Since then it has
been visited by numbers, and lately a steam-boat has regularly plied to
it twice a week, from the little town of Oban, in Argyleshire.
It was in this way that I went, and a most delightful trip we had
of it. The steam-boat left Oban in the afternoon, full of kind
light-hearted people, who seemed fully resolved to enjoy themselves. In
the evening we reached the island of Mull, where we were to spend the
night.
The inhabitants of Tobermory, (the little capital of Mull,) anxiously
look out for the arrival of the steam-boat, for they are very poor, and
the little money which the passengers spend, is a great object to them.
On these occasions the inns are seldom large enough to accommodate all
the strangers, and then a strange scramble takes place to get lodgings
for the night. Most of our party succeeded in doing so, and the rest
remained on board the steam-boat.
We went to see a beautiful little estate on one side of the harbour of
Tobermory, which belongs to the son of the old Laird of Coll, whom Dr.
Johnson visited. The grounds are nearly in the shape of a triangle, and
the two sides that are away from the sea, are formed by lofty rocks
with several beautiful waterfalls; and in the middle is a pretty lake.
Except the house on this estate, there are scarcely any good houses in
or near Tobermory, and the inhabitants are very ignorant and poor. Many
of them who could not speak any other English words, ran after us to
ask us for pence and tobacco.
We started early in the morning, and passed several small islands of
curious forms, composed of basalt; one in particular, which is called
the Dutchman's cap, shaped like this.
[Illustration]
As we approached Staffa, on the north side, we could see scarcely
anything of the pillars. It appeared a mass of Basalt, of very
irregular shape, rising abruptly out of the sea, scantily covered with
grass on the upper surface.
[Illustration]
It is the south side of the island that is chiefly remarkable, of which
I will show you a picture that I made on the spot.
It has (as you may see) exactly the appearance of a great layer of
earth swelling out at the edges, resting on a vast number of pillars
stuck close together. The colour of the pillars is nearly black, and
that of the stratum above, lightish green, yellow, and brown, from the
grass and variously coloured lichens that grow upon it. The sea is very
deep, quite close to the cliffs.
The day I was there, was as fine as possible; not a cloud was to be
seen, and the great ocean was as calm as a mill-pond, which is not very
common in these parts, for there is mostly a considerable swell.
The celebrated cave of Fingal, as it is called, (but why, nobody
knows,) is close to one extremity of the south side.
[Illustration: Plate IX.
FINGAL'S CAVE]
It seems just as if it had been formed by cutting away the middle of
some of the pillars, since you can see that many parts of the pillars
now remaining on the roof, are placed as if they were continuations of
the stumps on which you walk at the bottom.
We went into it in a boat, and when we were at the farther end, we got
out upon the broken pillars and surveyed everything at our leisure.
The length of the cave is two hundred and fifty feet, the breadth about
forty, and the height above one hundred feet at the entrance, and
seventy at the inner end.
Along the middle of the roof, is a deep cleft, or fissure, which makes
it something like a pointed arch, resembling the roof of a cathedral.
The sides of the fissure are variegated, yellow, red, brown, and white,
in consequence of water containing various substances, soaking through
from the surface above; and on each side of it, there are several rows
of the top of the broken pillars, which look quite black.
On the sides at the bottom, the stumps form a sort of pavement like
this, which is an exact copy of a small portion of it, and will show
you the forms of the pillars; some stand higher than others; but there
is not generally sufficient difference to prevent you from walking over
them.
[Illustration]
Between these pavements the water is very deep, and as it flows direct
from the Atlantic Ocean without any dirty shore or shallows, it looks
very beautiful,--of a clear emerald green, showing at the bottom
the black basaltic pillars in ruins, and a very few long luxuriant
sea-weeds gracefully waving with the undulations of the water.
The exposed surfaces of the pillars, between high and low water-marks,
are covered with the little shell-fish called _balanus_, or BARNACLE,
of colours varying from pink, which is the hue of those that are placed
deepest, to yellow and white, which are those that are least covered by
the water.
The walls of the cave above these shells, are of a deep slate colour.
The pillars which compose them, are on the average about three feet
in diameter, and they are fitted so close together that you cannot get
a penknife in between them. If separated at certain intervals, they
break short off, and leave a remarkably level surface without the least
splinter. Between these joints they break roughly and irregularly.
Here is a view of the inside of the cave.
It is supposed that these wonderful pillars are formed by the cooling
of a melted mass, and that this and other basaltic spots, are the
remains of streams of lava from a volcano long ago extinct.
In some of the lava streams of Mount Vesuvius, there has been noticed
an approach to this structure, though not quite so regular as what we
have been describing.
You may understand in some measure how it may come about, by examining
starch, which always hardens into little columns or pillars; and if
you look at the little sketch of a portion of the pavement of the cave
which I have given you above, you will see that nearly all the pillars
have six sides, the same as the pieces of starch.
It is remarkable what pains the basalt seems to have taken to get into
a six-sided form. I should think about two-thirds of the columns have
six sides, though sometimes one side is little more than a sharp edge
blunted, because it seems to have been prevented by its neighbour
making a similar effort to get its six sides complete.
None of the remaining third of the pillars, have more than nine, or, of
course, less than three sides. You might look a long time before you
would find two pillars exactly alike.
All this you will find occurring just in the same manner in starch,
except that the edges of the basalt pillars are quite even, while those
of starch are more or less waved or twisted.
Near the side of the cave there is a little rock formed by pillars into
a pretty regular cone, which you may see in the plate. It has a very
striking appearance as you approach the island.
Close by the rock there is a little cave, the mouth of which is formed
by bent pillars, in this manner.
[Illustration]
The water in this is not very deep, but it was deep enough to give one
of my companions a good ducking as he was trying to reach a shell which
one of the ladies of our party wanted to possess.
You will also often see something like these bent pillars in starch.
THE GIANT'S CAUSEWAY.
On the north coast of Ireland, the Giant's Causeway is a collection of
similar pillars of rather a smaller size. They stretch far into the
sea, and at low water you may walk a long distance upon the tops of
them.
The pillars get higher as they approach the cliff, and in the cliff
itself, just above them, is a remarkable layer of fossil wood in the
state called lignite, which closely resembles charcoal.
At some distance from the Causeway, some of the pillars stand up by
themselves, looking just like tall chimneys. There are also some
wonderful caves in the Basaltic cliffs behind it, but none of them
consisting of pillars like the great cave of Staffa. It is called the
Giant's Causeway, because some people have fancied that it resembled
the commencement of a great pier or causeway, which some beings of
superhuman power had left unfinished. This is only a fancy for poets to
talk of; you and I are now engaged about facts.
There are some similar formations in Iceland, of very great extent, and
in several other parts of the world; but those I have told you about,
are the most remarkable which are known to exist.