Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/373

 mile, mark the places to which the granite extended before it curved down to the N. & S., as before mentioned. At Wattern tor (fig. 2),

Fig. 2. — Wattern Tor, from the South, showing the decay along the lines of North and South Joints.

the chief mass is crossed by two north and south lines, along which decay has acted, dividing the upper part into three masses, and forming one of the most imposing tors on the moor. To the N.E. of this tor there is a narrow ridge with low cliffs on both sides, formed by parallel N. & S. lines of joint ; and at one place, in consequence of decay along the E. & W. joint, the storms have forced a passage, whence the tor has derived the name of Thirlstone. The E. & W. joints here are not perpendicular, but incline towards the south ; at Honey-bag Tor, near Widdicombe, in the moor, the inclination is towards the north. Near Manaton, on a hillside, there is a solitary tor, " Bowerman's Nose," rising about 40 feet above a " clatter of rocks ; " its prismatic shape is probably derived from the intersection of lines of joint. These tors have been selected from many as a series of examples of the effect of lines of joint, tracing it from the large massive cluster of rocks to the insulated pillar. Belstone Tors in the hard granite, and Hound Tor in the softer granite, are probably two of the best points for studying the hues of joint, and the different varieties of bedding, and decay along the lines of bedding, that occur in the east of Dartmoor.

It has been before noticed that the granite at the northern part of Dartmoor is more crystalline and compact than that to the south of the North Teign ; in the last-named district is the broad belt extending across Dartmoor mentioned in my paper on Rock Basins, as containing those hollows (Q. J. G. S. xv. p. 16). In this south- eastern division, in addition to the other sources of decay, is one that arises from the spheroidal structure of the granite. Professor Warington Smyth, in his Presidential Address for 1868 (page 1.), mentions that in Palestine blocks of granite decay from the centre to the surface, giving rise to rounded hollows. That species of