2. General sketch of the Structure of the Country between Dartmoor and Land's End.

It is well known, that in Dorsetshire and the south-western parts of Somersetshire, we meet with rocks of much more recent date than any I have yet mentioned. Even as far west as Haldon Hills, on the road from Exeter to Plymouth, we find an alluvial cap containing numerous fragments of common chalk flint; under which, are the remains of the green sand formation and of the beds with which it is usually associated.

From this elevation we command, on the eastern side, the rich woodlands of the valley of the Ex. On the south-west we have a country equally diversified and of the same general aspect, though less exuberant in vegetation. To the west and north-west, however, the face of nature is completely changed. A succession of ridges, rising to an elevation considerably greater than any of the neighbouring hills, exhibits a singularly broken and ragged outline; and the whole appearance of the contiguous country forcibly impresses us with the idea of its barrenness and desolation. Almost any one, while surveying the outline presented by these elevations, though still seen on the distant horizon, would be led to conclude that this extremity of Dartmoor was of a composition entirely different from any of the hills he had left behind him.

At the eastern end of Dartmoor commences the great granitic ridge, which prevails, though not without considerable interruptions, through the mid region of Cornwall, and at length terminates in the broken cliffs of Land's End. By whatever side we approach the moor, we find it flanked by hills of less elevation and of entirely different structure. The whole upper surface may be considered as making a rude approach to a broken table land of elliptical form, the longest diameter of which is more than 20 miles, and may be represented by a line drawn from Harford to Oakhampton.

From many of the higher parts of its western extremity we have a commanding view of the rich extent of country, which descends to the banks of the Tamer and the Tavy. This beautiful and picturesque region gains a double interest from being contrasted with the barren uniformity of the moor. The schistose rocks to the west of Dartmoor, in some instances, reach the elevation of eight or nine hundred feet. They are intersected by numerous mineral veins which continue to be worked to great advantage, more especially in the neighbourhood of St. Mary Tavy and Beeralston. As I made but a superficial examination of these districts, and brought few specimens away of any geological interest, I shall not detain my hearers by any further notice of them.

After crossing the new bridge, between Tavistock and Callington, to the right bank of the Tamer, we again find the granite breaking out from under the slate; the junction of the two rocks being exposed to view about 150 yards down the river. The fundamental rock does not occupy a country of any considerable extent; so that we reach its termination, and again descend on the schist, at no great distance from its commencement.

In advancing to the west by the high moorlands which prevail throughout the middle of the peninsula, we find that the most elevated portions of the tract are composed of granite. The ridge to the south-east of Camelford, the extended moors to the north of St. Austell, and the granite tors near Redruth, are examples of the truth of this assertion. A few miles from the last-mentioned place the granite tract deflects a few degrees to the south of its general bearing and abuts in the cliffs, between Porth Levan and Peran Uthno. After descending to the comparatively low lands, which extend to Merazion and St. Ives, we again find the soil resting on schistose rocks. But the broken outline and rugged surface of the hills farther west, plainly point out the re-appearance of the granite, which occupies nearly all the remaining extremity of the county.

With the exception of the tracts already enumerated, almost all the country extending from Dartmoor to Land's End, and bounded to the north and south by the Bristol and English Channels, may be referred to one formation. The prevailing rocks of this formation are known in the West of England by the provincial term Killas. When used by a miner, the term may designate slaty rocks, which differ from each other in colour, hardness, and almost every physical property. He uses it merely to distinguish the formation of which we are now speaking, from the various crystalline mineral aggregates which are known to him under the appellation moorstone, or growan, and which in hardly any instance exhibit a

fissile texture.

That granite is the fundamental rock of the whole region, cannot admit of doubt. We are not left to form this opinion from analogies deduced from the relative situation of similar rocks in other countries. In passing from the slate to the central masses of granite, we uniformly find the former rock rising up towards the latter, and reposing on it in a position which would generally be considered as conformable. If then we make the tour of any of the granitic tracts, we have an opportunity of examining such phenomena in detail, and may remark the schistose beds dipping in succession to every point of the compass.

A section of the country (from Cape Cornwall, through St. Michael's Mount, to Trewavas Headland between Merazion and Port Leven) which I have now the pleasure of exhibiting to the Society, presents no fewer than six beautiful illustrations of the relative positions of the two rocks.

The evidence for the fact I am endeavouring to establish does not rest here. Numberless excavations have been formed in the mining districts near the junction of the two rocks; and in various instances where those excavations have been carried to a sufficient depth; the whole superincumbent schist has been pierced through, and the operations afterwards carried down into the granite.

Such is the structure of the country I am describing: simple in its general outline, but presenting strange and perplexing phenomena to one who examines its features in detail.

At considerable distances from the granite, we often find the slaty rocks so contorted on the great scale, that it would be impossible to form any correct estimate, either of their dip, or line of bearing. The greater part of the county where they prevail is diversified by gentle undulations which are by no means so striking as those I have mentioned at the beginning of this paper. In some instances, the Killas rocks form the basis of considerably extended plains, which at a distance from the shore present the appearance of table-lands, terminated towards the sea by a broken line of perpendicular precipices. These noble sections of the strata prevail nearly throughout the whole coast of Cornwall; for there are comparatively few places where the cliffs exhibit such a shelving descent to the water's edge as to become the support of vegetation.

Before I enter on the mineral character of the rocks abovementioned, I shall take this opportunity of making one or two observations on the general appearance of the country. All the widely extended moors, more especially those in which the granite rocks predominate, are of a wild and dreary aspect. They are thinly covered with vegetation, and that often of the very worst quality; and in some instances nearly half the surface is occupied by granite bowlders, the remains of larger masses of the same kind which have gradually disappeared through the corrosive action of the elements. After descending from the granite ridge to the Killas, we often find a country almost destitute of foliage, and in few respects more inviting than the one we have left behind. In some of the mining districts, the whole surface is nearly covered with unsightly mounds of rubbish, which have been accumulating for centuries, and are so impregnated with mineral matter that a blade of grass will hardly vegetate in their neighbourhood. The rich

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tract which is so widely extended to the west of the Tamer; the banks of the Foy river; the lands bordering on the different reaches of the Fal; and the whole country along the extended margin of Mount's Bay, form the most striking exceptions to the general nakedness of the county.

One who is attracted by the grand and more rugged features of nature will find many of the high Tors and masses of decomposing granite, which lie scattered about the moors, well deserving of his examination. The extended line of cliffs, almost always precipitous, often broken into bold headlands and exhibiting in their forms the peculiar characters of the rocks which compose them modified by the progress of decomposition; affords numberless scenes not less delightful to the common traveller than instructive to the naturalist.

§. 3. Granite and the Rocks associated with it.

A crystalline aggregate of quartz felspar and mica forms by far the greatest part of the fundamental rock, in the region described in the preceding section. Varieties, arising from the loss of one of these ingredients, or from the addition of some other mineral, are by no means uncommon. As these however form the exception, any further consideration of them will be postponed till we have given some account of the more prevailing rock. Keeping therefore in mind its most general character, the granite of the west of England may be described as coarse-grained, and of a greyish or yellowish colour, derived from the felspar which is the predominating ingredient. When examined on the great scale, it is often found porphyritic; the three constituents forming a granular base in which are imbedded large crystals of felspar. We may observe also that these large crystals often exhibit a bright clear fracture, while the felspar of the base is dull, earthy and decomposing. The large prismatic crystals are not unusually of that structure which Haüy