Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/370

 is, as a whole, a coarse-grained mixture of quartz, felspar, and mica, the latter sometimes white, at others black, the two micas occasionally occurring in the same mass. It is frequently porphyritic, from the presence of large crystals of felspar, and here and there schorlaceous ; but the latter character is chiefly confined to the outskirts, where the Dartmoor granite adjoins the slates." This description is very applicable to the southern part of the district, which will now be noticed, where the granite is said greatly to resemble that of Auvergne ; but to the north of the river Teign the rock is more compact and crystalline.

The apparent stratiform appearance and rapid changes of character in the beds of granite are so well known to most geologists, that it is not needful to enter upon that point at length. Mr. D. Mackintosh, in his paper "On Oblique Lamination in Granite" (Q. J. G. S. vol. xxiv. p. 279), mentions instances of the linear structure of that rock on Dartmoor, and the occurrence of striking variations in its character in overlying parallel beds. De la Beche's report states that " a very general structure prevails throughout the Cornish granites, and is more particularly observable in the large masses. This structure consists in a division of the granite into portions resembling beds, which form tabular masses when they are extensive, the edges bending beneath the adjoining schistose rocks " (page 163). I have observed that the beds also occasionally bend or dip on Dartmoor. Thus, on the north-east side of the Moor, at Scarrey Tor and Higher Belstone Tor, on Belstone Ridge, the beds of granite dip towards the Carboniferous rocks which closely adjoin them on the north-west. The last-named Tor is cut off abruptly at the south-eastern end, and the side of the hill is for a considerable distance covered with angular fragments. Possibly the granite originally dipped also to the south- east, and the beds broken up by the joints have rolled down to the river Taw. Similar curvature has been noticed in places remote from the schistose rocks ; thus, on Teigncombe Common, near Chagford, more than two miles distant from the carbonaceous rocks, the granite at Kestor dips in a northerly, and at Middleton in a southerly direction — causing probably the contour of the hill, and the valleys of the north and south Teign. At Blackingstone, near Moreton Hampstead, the beds curve down on the north side, probably causing the valley that lies between that rock and the White Rock, where the beds lie nearly horizontally. At Houndtor (about halfway between Chagford and Ashburton) the beds at the west end are nearly horizontal ; at the east end they curve downwards, and probably cause the valley between that and Leign Tor. These are the most obvious examples of " dip" in the granite that I have noticed in the district to which this paper relates ; in all these cases the dip is very perceptible, and quite distinct from oblique lamination.

The same Report states (page 165) that " it is the intersection of the more or less perpendicular joints or divisional planes with the stratiform structure in the Cornish and Devon granite which gives it the appearance of being composed of a multitude of rectangular