304 



THE GEOLOGIST, 



ject are also well brought out to view, and exposed for solution by 

 subsequent observers. Not much, however, has been done since the 

 date of that memoir. Mr. Ormerod's communication on the Rock- 

 basins of Dartmoor, in the Geological Society's Journal, 1858, vol. 

 xiv., p. 16, &c., contains a resume of our present knowledge on the 

 subject, and to this we shall presently recur. Fig. 5 represents one 

 of the Dartmoor tors, the granitic boss kno^^m as Blackistone ; and 

 fig. 3 is a sketch of Haytor (eastern side), also in Dartmoor. The 

 latter is from my note-book ; and, in connection with some appear- 

 ances on the faces of the granite-quarries on the lower ground to 



Lign. 5.— Blackistone, Dartmoor. 



the westward, offers an interesting illustration of the method of the 

 formation of "tors," " cheesewrings," and "logging stones." 



A few hundred yards below Haytor is an old quarry, the smooth 

 faces of which are formed of the joint-planes of the granite. These 

 faces show many of the cross-joints ; and on the eastern face the 

 north-and-south perpendicular joints are plainly seen here and there 

 to be the channels by which the rain and frost are working their 

 way into the rock, producing innumerable minute cracks close to 

 and mostly parallel with the gTeat fissures (fig. 6). This is especially 

 the case beneath the surface-soil, where the joints are enlarged into 

 a compound group of cracks, minutely cutting up the stone into 



