The geological notices inserted in this Work, are necessarily so brief
and so unconnected, on account of the alphabetical arrangement of the
parishes, that it is desirable that a few introductory remarks should
be made that this defect may be in some measure obviated.
Cornwall does not possess a great variety of rock formations, being
composed of primary and transition or intermediate groups, covered
here and there with deposits of gravel, sand, and clay, which belong
to the modern epoch. But this limited range of formations is more than
compensated for by the great facilities which the geologist here
enjoys in his investigations; the very extended line of cliffs which
deeply indent the Cornish shores proffer numerous and instructive
sections; and the vast mining operations have brought to light most
important phenomena. Indeed, no country of equal extent can, in these
combined advantages, be compared with Cornwall.
On a general view of this country, the surface exhibits two systems of
valleys; one running parallel with the central ridge, which is highest
near Launceston, and gradually, but irregularly, declines till it
terminates at the Land’s End, in cliffs about a hundred feet in
height; the other intersecting the longitudinal valleys at right
angles; and as all the intermediate hills are more or less rounded,
the country has an undulating appearance in both directions, not
unaptly compared to the waves of the sea. Through these two systems of
valleys, the rivers flow, seeking outlets into the sea by the nearest
continuous descent; sometimes they effect this along the longitudinal,
at others through the transverse valleys, receiving tributary streams
on either hand from the lateral valleys, or curved hollows which they
intersect. But sometimes the river is diverted in its course more than
once, in consequence of the concavity of a cross curve rising above
its level――or by that of the other system descending below this
point――by which irregularity the stream is compelled to flow along the
more favourable drainage; thus the Camel rises about four miles from
the sea, in the moors near Davidstow, and flows more than twenty-five
before it empties itself into the sea at Padstow. In the hollows of
these valleys the drainage is often sufficiently obstructed to produce
marshy ground; but seldom of such extent as to deserve the name of a
lake.
As regards the soil, on the high grounds it is frequently very shallow
and barren; but in the valleys it is very productive, and here and
there is well wooded, more particularly on the southern coast. On the
northern coast, in the vicinity of the sea, it is very sandy, owing to
the light testaceous sand of the shore being carried inland by high
winds; in this manner no inconsiderable tracts of fertile land have
been devastated.
Let us now turn our attention to the internal or geological structure
of Cornwall. The highest parts of the central ridge, already alluded
to, are composed of granite, which occurs in the form of four large
insulated patches, so disposed at nearly equal distances from each
other as to resemble a chain of islands extending from Launceston to
the Land’s End, that is in the direction of N.E. and S.W. On the same
ridge, but rather parallel than continuous thereto, is the great
granitic group of Dartmoor in Devon, the whole of which is sometimes
called the Ocrynian Range.
The granitic patch of Dartmoor is by far the most extensive, being
nearly twenty miles in diameter; that of Launceston is ten miles in
length by six or seven in breadth at its widest part; and its most
elevated hills, Rough-tor and Brown-Willy, do not much exceed 1,300
feet in height. The granitic rocks of this patch, like that of
Dartmoor, are not much exposed by artificial excavations, so that
their varieties cannot be easily examined; the _weathered_ blocks,
which on the summit and sides of the hills form tors and detached
masses, consist of the hard or siliceous varieties of the common and
fine-grained granites, such as have withstood the action of the
elements. One of the most curious of these tors is the _Cheese Wring_,
near Liskeard, a pile of single blocks, each being larger than the one
immediately beneath. Proceeding along the ridge towards the Land’s
End, the next, or St. Austel patch of granite will be found, much less
than the last, but more interesting both in a scientific and a
commercial point of view. In addition to the kinds of granite already
noticed, it contains beds of talcose granite, or protogine, which by
its decomposition furnishes that valuable substance _china clay_ or
_kaolin_, many thousand tons of which are annually exported for the
potteries. The third, or Redruth patch of granite, affords many
varieties of this rock, and has been well explored by numerous mines
which have been productive in both tin and copper ores, affording also
to the mineralogist a great variety of rare specimens. But the fourth,
or Land’s End granite, is by far the most important to the geologist,
for the land becoming here very narrow, the sea has produced
cliff-sections, both in the granite, and also at its point of junction
with the slate, exhibiting many interesting phenomena. Among these,
the veins of granite in the slate are beautifully displayed, and have
long been a great attraction to geologists; but the modes in which
these rocks meet and unite, are not less deserving of notice. But for
a detailed description of these curious facts, the reader may be
referred to my “Treatise on Primary Geology.”
Besides these four principal patches of granite, there are four
others: 1st. that of Kitt Hill, near Callington; 2nd. that of
Tregonning and Godolphin, near Helston; 3d. that of Cligga Point, near
St. Agnes; and lastly, that of the celebrated St. Michael’s Mount. The
two first are of some size, and at the surface are both subdivided
near the middle, by the overlapping of the slate: the other two are
small, but very interesting to the geologist.
It is worthy of remark that the fertility of these granitic groups
gradually increases as they diminish in elevation; and it is a curious
but not surprising coincidence, that the number of parish churches
thereon follows the same order:――thus on the eastern and most
extensive tract of granite, near Launceston, there is only one church;
on the next there are three; on the Redruth patch six; and on the
Land’s End granite no less than nine, within a space considerably less
than that of the eastern tract.
Each of these insulated groups of granite is surrounded by schistose
rocks, the layers of which, on all sides, incline from the granite at
various angles, from 20° to 40°. Although these groups are thus
separated from each other by the slate at the surface, yet it is the
general opinion that they gradually approach beneath, until they are
all united into one and the same mass――the intermediate hollow spaces
(the valleys, as it were, between the granitic mountains) being
occupied by the slate. It might, however, be maintained that the
granite is imbedded in the slate, in large rounded masses, which would
also account for the former rock underlying the other, within the
limits of mining operations: and such an opinion would derive some
support from the fact, that small insulated masses of granite in the
slate are not of unfrequent occurrence.
The slate formation consists of two very distinct groups, when the
most characteristic rocks are alone regarded; but it is not easy to
trace the boundary between them, as the contiguous rocks appear to
pass gradually into each other. These groups have one character in
common: viz. that they consist of several distinct kinds of rocks,
each genus of which respectively may be subdivided into schistose and
compact rocks; the latter are imbedded in the former――are more
crystalline――and often contain clusters of their constituent minerals:
so that the complicated composition of these rocks is made known by
Nature’s own analysis.
The group next the granite is _primary_, the more remote one is
_transition_, or as they are termed in the following pages the
_porphyritic_ and _calcareous_ series of the slate formation. The
series next the granite is characterised by its beds of porphyry, and
by its abounding in veins of tin and copper ores; the other series by
the frequent occurrence of calcareous spar and strata of limestone,
with occasional organic remains, and by its being sparingly
metalliferous, containing no tin-ore, but being productive of lead and
antimony. It is within the last series that the magnesian or
serpentine rocks occur, and which in the Lizard district are developed
to a large extent, forming one of the most interesting geological
features of Cornwall. Those who are desirous of more information
concerning this slate formation, will find a copious account in the
fourth volume of the Transactions of the Geological Society of
Cornwall. It is there proposed to give these rocks, whenever they
possess well-marked characters, distinct names, and not to refer a
great variety of rocks to the vague and indefinite genera of
clay-slate and greywacké. An accurate and more extended nomenclature
would have rendered the following notices much more intelligible.
In describing the general features of Cornwall, we must not omit to
mention the metalliferous veins――the great source of its commercial
prosperity――and the channel through which much curious information has
been obtained, concerning the structure of the earth. These veins
traverse indifferently both the granite and the slate, but are most
abundant in the latter rock, in the vicinity of the granite. The
general direction of the tin and copper veins (or _lodes_, as they are
provincially called) is nearly E.N.E. and W.S.W., and they are crossed
by another system of veins, nearly at right angles, which are not
commonly metalliferous, and when they do contain ores these are often
of lead, antimony, silver, and other metals. This is not however a
general rule, for in the parish of St. Just, Penwith, the tin and
copper ores occur in the cross veins. The course of the veins is not
straight, but they are always more or less undulating, both in their
direction and in their dip or underlie. Various interesting phenomena
result from their meeting with and intersecting each other, known to
the miner under the names of _heaves_, _throws_, and _slides_. This
subject is replete with curious facts, but would require considerable
space for their enumeration; the reader must therefore be again
referred to the work already alluded to on “_Primary Geology_.”
Before concluding this brief sketch, a few words must be said on the
beds of clay, sand, and gravel which cover the low grounds, both on
the granite and on the slate. These may be resolved into two kinds:
1st. those resulting from the decomposition of the rocks, and which
are in their original position; and 2d. those which are not _in situ_,
affording by their appearance, composition, and position, evidence of
their having been transported. These deposits belong to three distinct
periods, and alternate with terrestrial and marine remains, according
to their situation in respect to the sea-level at their respective
epochs. Each of these transports was accompanied, or followed by, a
change in the sea-level: the last corresponds with that now existing;
the previous one, by the elevated ancient beaches, indicates that the
sea was about fifty feet above its present level; and the earliest
transport, being covered with sub-marine forests, shows that the sea
was at that epoch at least fifty feet lower than it now is. (See 4th
vol. Geol. Trans. of Cornwall, p. 466, et seq.) In the lowest or
oldest deposit the tin-ore (_stream-tin_) is found in the form of sand
and gravel mixed with earthy substances: it affords the purest kind of
tin, known in the market by the name of _grain-tin_; and appears to be
the source from which the ancients derived all their tin. The notices
under each parish will furnish more particulars concerning these
interesting deposits.