Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/371

 blocks ; " and the observations of Professor Sedgwick and Mr. Enys are there quoted (page 171) to show that the general direction of the lines of joint in Devon and Cornwall is from N.N.W. to S.S.E., and that on the northern part of Dartmoor the granite is also cleaved in lines at right angles to it, or nearly so. Variations from these directions are often seen, and the E. & W, joints occur along the whole of the eastern escarpment ; but my own observations agree as to the general direction of the lines of joint with those above quoted. The general and most simple appearance of a joint is merely that of a crack passing nearly perpendicularly through a rock which is of similar character on both sides ; but the variations are frequent ; and of these a few instances will be given. The joints vary in width from a line to an opening of several inches, where the decay has been great ; and in mineral veins, if they ought to be regarded as occupying joints, the width will often be several feet. The joints vary from the perpendicular ; and the east and west joints incline more than the north and south ; and there is no rule as to the direction of the inclination. Sometimes there is a difference in the character of the granite on the opposite sides of a joint. A vein of decaying granite occasionally occurs between compact masses of granite of similar character ; a vein at Carrion Hill, near Moreton Hampstead, is an example of this ; but veins of decaying granite, with hard rock adjoining, often occur, which are not apparently connected with joints. An example may be seen in the cutting to the south of Lustleigh station. A series of parallel joints may occasionally be noticed with a few inches only of rock between them ; and though these portions of rock are alike in general character, the felspar crystals in each will be placed in different positions. The connexion of joints with mineral veins will not be entered upon, except to notice that the joint not unfrequently passes nearly along the centre of a vein of felspar, where it is marked by a black line of fine schorl, in which case the felspar is of a pink colour. Several of these variations may be seen in the weathered cutting on Mill Hill, near Chagford. The character of the granite by the side of the joints (as mentioned in Report, page 190) occasionally appears to be altered; on the surface it is black, often from schorl, and presents a tessellated appearance, caused by minute cracks, which penetrate about an inch deep into the rock. If this coating is removed, the unaltered granite of the district will be seen below. Examples may be seen near Chagford, at Lower Murchington, Mill Hill, and about a quarter of a mile from the town by the side of the road to Moreton. Minor lines of joint, extending only over small areas, occur occasionally. These cross at rather acute angles ; and when the sides of these minor lines of joint have been altered as above mentioned, the granite presents a basaltiform appearance, which, if not carefully examined, may mislead. A good example may be seen at the base of the north end of West Hey Tor, where the basaltiform beds form a stratum only a few feet in thickness, but many of the joints extend upwards into the overlying rock. These joints are perpendicular.