338 ROCKS OF IGNEOUS ORIGIN ON DARTMOOR. [Aug. 1 894. 



22. Notes on some Trachytes, Metamoephosed Tuees, and oilier 

 Rocks of Igneous Origin on the Western Flank of Dartmoor. 

 By Lieutenant-General C. A. M c Mahon, F.G.S. (Read April 

 11th, 1894.) 



During the last two seasons I have spent some time in the examina- 

 tion of the rocks of igneous origin which occur between Lydi'ord and 

 Okehampton, and which are beyond the area dealt with by Mr. Frank 

 Rutley, F.G.S., in his memoir on the ' Eruptive Rocks of Brent Tor 

 and its Neighbourhood.' As these rocks do not appear to have 

 been described, some details regarding them may be of interest. 



On the Geological Survey map a long outcrop of ' greenstone ' 

 is depicted running from Great Cranaford, to the east of Sourton, 

 almost as far as the West Okement River. The present paper is 

 principally concerned with the outcrops of this ' greenstone ' on 

 Sourton Tors, South Down, and Meldon, and it will be found that 

 they embrace a considerable variety of rocks. 



I. Sourton Tors. 1 



Sourton Tors form a ridge to the south-east of the village of Sourton 

 (see the accompanying sketch-map). It rises to the height of 1447 

 feet above the sea, and 650 feet above the village. The rocks along 

 this ridge crop out in two distinct lines ; one of these — that on the 

 western side of the ridge — forming a series of disconnected outcrops 

 which I have numbered 3, 4, 5, 10, 11, and 12 ; and a second or 

 eastern line, which I have marked 2, 6, 7, and 8 on the accompany- 

 ing map. The space between these two lines is covered with grass, 

 and the rocks between them consist, it is to be presumed, of sedi- 

 mentary rocks of the Carboniferous (Culm) series, similar to those 

 seen along the line of strike in the railway-cutting to the S.W. of 

 the Tors. No. 1 (of map) is on lower ground, about 447 feet below 

 the highest point of the Tors, and is lithologically connected with 

 the second outcrop. No. 9 (of map) is described in detail farther on. 



The sedimentary rocks along the road between the villages of 

 Lake and Sourton- — namely, on the western side of the Tors — dip 

 N.N.W. In the railway at the Lake end of Sourton Tors, the strata 

 dip about 60° to the W.N.W. 10° N. On the eastern crest of the 

 Tors there is a small projection of a calcareous sedimentary rock 

 dipping N.N.W. 10° N. I am not certain, however, that it is in 

 place. But on the north-eastern flank of the Tors the sedimentary 

 carbonaceous rocks crop out in situ, near the boundary of the Dart- 

 moor granite, on the left bank of a stream running down from 

 Sourton Tors to the West Okement River. The dip is here N.W. 

 10° W. 



The general dip of the rocks which form Sourton Tors may, I 

 think, be taken to be about N.W., and if so, the outcrop of the 

 rocks of igneous origin, about to be described, conforms to the dip 

 of the sedimentary beds. 



1 This is written Sourton Tor (in the singular) on the one-inch Ordnance map. 



