Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/375

 rocks near the Teign north of Moreton, and the Parson's Brown Loaf near Lustleigh Rectory.

In noticing the effects that bedding, joints, and spheroidal structure have had, and now have, it has been necessary to touch on each separately ; but it is scarcely needful to say that these three causes are constantly acting together. The action of joints in dividing the masses which have since become isolated, and of decay penetrating between the stratiform blocks, have doubtless shaped the rocks that form the Cheese Ring and similar places. To the joint action of all three causes may probably be attributed the Logan or Rocking Stones of the east of Dartmoor, at Belstone, and Thornworthy, and to that of bedding and joints, probably, that on Rippon Tor ; the stones that form these three Logans are probably in situ. The Drewsteignton Logan in the Teign is of compact, hard, angular felspathic granite, and rests upon granite ; it is in the Carbonaceous district, about a quarter of a mile to the east of the point where the Dartmoor granite ceases : it is not in situ ; and the amount of large felspar crystals contained in the stone lead to the opinion that it has not formed part of any of the numerous granite and elvan veins which in that locality penetrate the Carbonaceous rocks. The well-known " Nutcracker," the Logan in Lustleigh Cleave, is on the side of the hill, and is not in situ ; it is apparently a block of granite which has rolled down from above and fallen into a position in which it could be easily moved. This Logan was wantonly thrown out of balance a few years since, but has now been replaced. Another Logan of similar character is almost close to the " Nutcracker." A further examination has shown that the rocks at Leigh Bridge described by the author at p. 419 of vol. xxiii. of the ' Quarterly Journal ' as " masses of granite piled on each other," are in situ. This appearance is due to the north and south lines of joint and spheroidal structure.

In this paper attention has been directed solely to the bedding, joints, and decay from spheroidal structure which appear in times past to have been, and which now are, active agents in producing the disintegration of the granite, of which the effects are so apparent; doubtless the varied manner in which the rocks have been acted upon arises in a great degree from the peculiar mineralogical character of each district ; but this is a point on which I much regret that it is not in my power to give reliable information.

It has been needful occasionally to go over ground which has already been partly described by others ; and in those cases it has been endeavoured to incorporate the opinions and observations of the various authors in their own words.

Discussion.

Prof. Ansted had observed similar conditions in Leicestershire, Alderney, and elsewhere. The most important feature in the case was the amount of subaerial disintegration and denudation to which the rocks, and especially the Tors, bore witness. The bedding