1.-GEOLOGICAL SURVEY OF SOME PARTS OF THE

COUNTRY NEAR PLYMOUTH, PARTICULARLY BETWEEN THE PLYM AND TAMAR; BY JOHN PRIDEAUX, MEMBER OF THE PLYMOUTH INSTITUTION.

THIS limited, and possibly inaccurate, geological sketch of our neighbourhood, will, I hope, be acceptable to the society, both because the variety of rocks in so small a space, the diversity of their position, and their extraordinary exposure and accessibility, are of sufficient interest to deserve being known; and that it is well for institutions like ours, to be acquainted with the details of whatever concerns science in our own precincts, to lighten the labours of those who investigate on a broader scale.

The granite of Dartmoor, from Mist-tor to Hey-tor, and south of that line, has the following characters; which probably belong to the whole.

It is entirely mountainous, the highest hills being on the borders; where some of them attain an elevation of nearly 2000 feet. The valleys run in various directions; but have a

tendency upon the whole, to the north and south line. The hills rise often steep, sometimes precipitous; their sides scantily clothed with long grass, except where rushes or moss indicate subjacent bog: and often strewed with loose blocks of granite, from 50 or more tons down to the size of a flagstone. A crag, called the tor, usually projects at the summit of the hill, having a very striking appearance of stratification; the fissures being sometimes horizontal, more commonly a little inclined. This stratified character is not less general in the quarries; where, although there are none of those marked divisions, indicative of intermissions in the original deposition of the rock, the stone always comes out in beds. The dip is different in different hills, but seems to have a prevailing tendency toward east and south.

Not only the hills are higher on the borders, but in many places the granite seems harder there, and of closer texture. Hey-tor, Sheeps-tor, Collard-tor and Pen-beacon will illustrate this; as well as Calstock, the granite of which belongs to a more westerly projection. We must except, however, as will présently appear, the hill sides in immediate contact with certain rocks adjoining the moor.

The hardness varies, from such as almost to defy the tool, to that which falls to pieces by the blow of a hammer, or may even be cut down with a spade, like gravel.

The colour is not much more uniform, being generally pale grey or whitish in the mass, with a shade of red or yellow; but it is found, from almost black with schorl, to pure shining white: and some occurs of a rich red, superior in beauty to any Egyptian granite I have seen; particularly where it contains tourmaline.

That it is metalliferous every body knows: tin being the most common product. Copper sometimes occurs, and in a few

instances, manganese has been mentioned; of which, as the miners have reasons for concealing it, I do not know the geological relation to the rock-whether it lies on the surface, or penetrates it in veins. Lead I have not heard of in this formation.

This granite is rich in schorl and poor in mica, consequently less impregnated with magnesia, and perhaps more subject to the operation of the weather than is common to that rockcircumstances which may help to explain its comparative fertility in grass. In summer it feeds great numbers of cattle; and in extremely hot and dry weather, when the herbage elsewhere is burnt up, assumes the appearance of great verdure; its humid soil and cold atmosphere, which at other times give it a pale and hungry aspect, contributing then to its fertility.

The close crystalline texture of the rock, obliging the condensed fogs and rain to run over its surface, is probably the cause, that the valleys are boggy; extending in numerous cases, up the hill sides and across the summit. These bogs, dangerous to cattle, are not without their value; being the great depositories of fuel; an important article in those bleak regions, where a fire-side is often agreeable in an evening of July. The peat gradually accumulated there, has attained a depth in many places, exceeding 20 feet. The deeper, the more it is prized: the lower portions, condensed by the superincumbent weight, becoming a much more effective fuel.

Springs commonly break out from these bogs, as though superficial to the stone; but there are intances of a different kind on the eastern foot of Ugborough-beacon, is a fine spring, pouring out of the rock, probably two hogsheads a minute. Such as these may perhaps owe their origin to another condition of the granite, where the crystals of quartz and felspar are incoherent, and the schorl has very much

disappeared, forming a sort of gravel many yards in depth. Such a bed of great extent, lies on one side of Hessary-tor, near the prison.

In wood, this rock appears to be unproductive. A few young plantations of fir do not yet appear to suffer more than might be expected from the climate; some fine trees are found about the borders of the streams; and trunks of considerable dimensions, have been dug up from the bogs: but it is said by gentlemen possessing estates on the granite, and my observation agrees with it, that trees, after reaching a certain height, rise no farther; spreading and twisting their branches, without proportionate increase of trunk. Wistman's-wood, a plot of oaks, supposed to be of a thousand years standing; the largest less than a man's waist, and within twenty feet high, is an extreme instance.

The outline of the granite, from Tavistock to Hey-tor, southward, may indeed be almost traced by the coppice, which clothing the declivities of the slates and other rocks that abut against it, disappears suddenly on its gritty soil.

Entering the moor from Tavistock, Cocks-tor, a trapp mountain must be passed, before we reach the granite at Staple-tor. Thence it runs a little westward of south to Pu-tor, two miles; to Crip-tor, east one mile; to Sheeps-tor, south five miles; whence it leaves a deep curve, occupied by a hill of schorl rock, with some slates, (Ringmoor-down,) and comes out west again at Wigford-down. At Dewerstone, the southern angle of this down, and S. W. point of the Dartmoor granite, it is finely exhibited in a vertical cliff of probably one hundred and fifty feet, down to the bed of the river. Hence E. S. E. about two miles to Collard-tor; N. by E. three miles to Pen Beacon; E. one and half mile, and as much S. to Blatchford-hill; and about S. E. by S. to the Western-beacon, full four miles. This is the southernmost

point; thence about two miles N. E. by N. to Ugborough beacon; about four miles in a concave line N. by E. to Shipley-tor; thence a little westward of north, about seven miles to the Dart. From Sharp-tor, on the north bank of the Dart; N. E. by N. about four miles to Widecombe; S. by E. about the same distance to Rippon-tor; and doubling back N. E. by N. three miles to Hey-tor; which is probably its easternmost projection. Mudge's map may be referred to for these points.

Gneiss has no where occurred to me, within this range; but the slate, where in contact with the granite, is often micaceous-as at Meavy, Shaugh, Black-alder-tor, Hey-tor &c. These micaceous slates extend but little way from the granite, clay slate succeeding; though usually separated by an intervening valley. At Black-alder-tor, the mica slate merely covers the granite like a peel. The only place I met with, where the soil on this slate was extensive enough for cultivation, is at Hey-tor; where it appeared to yield good crops of corn.

In other cases, as at Collard and Walkhampton, the slate in contact with the granite is not spangled; but has a general glimmering surface approaching to that of talc, without the unctuous feel of magnesian minerals. This slate is not unfavorable to agriculture.

On Yanadon-down; from the Western-beacon to Ugborough beacon; and in a larger tract from Dean to Holne, a hard stony slate bounds the granite. This rises into steep and high hills, unfertile, except in wood. It dips regularly away from the granite, at different degrees of declivity, but generally steep.

Great part of the way from Ugborough-beacon to Deanywood, and again on the north side of the Dart, the granite is