350 LIEUT.'-GEN. C. A. M c MAHON ON ROCKS OF [Aug. 1894, 



All the slices contain magnetite. Nos. 30 and 32 contain much, 

 and 33 a little anthophyllite with the habit already described'. 



III. South Down. 



On the left bank of the West Okement Eiver, opposite Meldon, a 

 ridge rises abruptly from the river, and stretches away until it 

 widens out into South Down and ultimately merges into Sourton 

 Tors. 



I may note in passing that I discovered, last year, a second 

 outcrop of the Meldon white granite on the eastern flank of South 

 Down. It is not on the spot where a second outcrop is marked on 

 the Survey map, which I stated in my last paper that I was unable 

 to find. That is placed on the right bank of the river, extending 

 in a northerly direction across to the left bank. The outcrop which 

 I found is away from the river altogether, on the eastern side of 

 South Down, some 300 feet above the West Okement Eiver. A 

 small quarry has been opened here, and the rock has exactly the 

 same appearance as that of Meldon. The latter is, as the crow flies, 

 about § mile distant in a north-easterly direction. 



In the Carboniferous slaty rocks that compose the ridge running 

 up from the West Okement Eiver to South Down, and at a lower 

 horizon than the Meldon limestone, a plagioelase-mica-hornblende 

 rock occurs which merits notice. It is a compact igneous rock of 

 purple-grey colour which has the appearance, in the field, of being 

 a contemporaneous lava. It runs with the sedimentary rocks ; and 

 although, owing to vegetation, its outcrop is not continuous, it is 

 always found under a dark-blue slaty Carboniferous rock, that, in 

 the field, is somewhat suggestive of a limestone. Though the mica- 

 diorite disappears from view occasionally, it can always be found 

 cropping up again farther along the line of strike. 



The Carboniferous beds dip N.W. 15° N. at the foot of the ridge ; 

 higher up they dip N. 20° W. This dip continues until a wall is 

 reached, which forms the boundary between the top of the ridge 

 and the beginning of South Down : here the rocks make a sharp 

 bend, the strike becoming N.E. 10 c N. On the top of South Down 

 the dip turns over to W.N.W. ; that is to say, it reverts approxi- 

 mately to what it was in the bed of the West Okement Eiver at the 

 foot of the ridge. 



The mica-diorite now described crops out first (specimen 35) on the 

 flank of the spur, or ridge, just above the river. It rises to the crest 

 of the ridge (specimens 34 and 36), and, owing to the sharp bend in 

 the strike of the rocks above alluded to, it passes along the eastern 

 side of South Down. I have not traced it beyond that point. 



In my last paper I pointed out that De la Beche's geological map 

 is not to be relied on too implicitly for the boundaries of the 

 ' greenstone '-outcrops marked on it. The case in point is another 

 instance of this ; a continuous outcrop is indicated from the West 

 Okement Eiver over South Down to Great Cranaford. No such 

 outcrop exists : the rocks of igneous origin visible along this line 



