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.