310 



THE GEOLOGIST. 



Ormerod, who have treated of these phenomena in the papers 

 abeady referred to. On the top of Haytor there are traces of two 

 or more of these shallow basins (fig. 9) . They are enumerated by 

 Mr. Ormerod in his Table of Tors and Rock-basins, op. cit., p. 26. 

 We cannot do better than quote at leno-th some of Mr. Ormerod's 

 observations on the formation of rock-basins, whence the reader 

 may learn m.uch respecting the physical character of the Dartmoor 

 granite. 



" Sir Hemy de la Beche in a note on the ' Report on Cornwall, 

 Devon, and West Somerset' (p. 452), writes: — 'Dr. M'Culloch has 

 suggested that the friction of the quartz and felspar-fragments not 

 unfrequently found in rock-basins may have contributed to deepen 

 them. As we have often observed these fragments in motion during 

 high winds, both when the basins were dry, or a small quantity of 

 water in them, we are inclined to believe that this may the case.' 

 The fragments occur in most basins ; in some, the bottom is covered 

 by them. Rolled stones similar to those which occur in the ' pot- 

 holes' have not been found by Mr. Ormerod in any basin, but the 

 contents generally consist of small angular fragments of quartz and 

 felspar, and schorl, which sometimes cover the bottom of the basin. 

 Small lumps of gi-anite occasionally are found, not rolled, but that 

 have a|)parently fallen in where the sides are much weathered and 

 falling to decay. Although in the habit of inspecting the basins in 

 every state of the weather, from the mildest breeze to the heaviest 

 storm, Mr. Ormerod had never seen these particles blown about in 

 the water in the basins having the bottoms flat and sides upright, 

 and had only seen them moved in shallow concave basins when dry, 

 or when a heavy gale had blown them out together with the water. 

 The cause suggested by Dr. M'Culloch could not affect the deep 

 basins, as in those cases the particles would be undisturbed by 

 motion of the water from ^dnd. These small fragments, however, 

 throw some light on the manner of the fonnation of the rock-basins. 

 The granite of the Dartmoor district is in a great measure porphy- 

 ritic ; it is for the most part of a large coarse grain, and schorl in 

 variable proportions frequently occurs ; globular nodules, varying 

 from an inch to upwards of a foot in diameter, often occui\ These 

 vary much ; sometimes they are harder than the adjoining rock. 



