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.”