Page:The parochial history of Cornwall.djvu/35

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xxvii
GEOLOGICAL INTRODUCTION.

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. Pro-