﻿Yol. 64.] .STRUCTURE OF THE ST. DAVId's AEEA. 373 



IV. The St. David's Geanophyee. 



Immediately south of St. David's occurs the disputed granitoid 

 rock, termed by Dr. Hicks ' Dimetian.' Its true nature as an 

 igneous rock was demonstrated by Sir Archibald Geikie in 1883; 

 and, as it has been frequently described, no more need be said 

 with regard to its petrographical characters than that it is of 

 variable texture, highly siliceous and coarsely granophyric, con- 

 sisting of quartz, ortboclase, oligoclase, a little microcline, chlorite 

 (probably after biotite), and ferruginous matter, with nests of 

 epidote. It weathers in a fissile manner, and decays, where there 

 is moisture, to a characteristic yellow colour. The quartz has a 

 dirty appearance, owing to numerous inclusions, often specially 

 developed along planes of cleavage. 



The granophyre frequently shows signs of crushing, and is 

 traversed by bands in which the rock is greatly shattered. 



V. Extent and Boundaeies of the Dimettan. 



According to Dr. Hicks, the Dimetian was of pre-Pebidian age 

 and all its boundaries lines of faulting ; the view taken by Sir 

 Archibald Geikie was that the rock was a granite of post-Cambrian 

 age and its boundaries, in the main, intrusive junctions. 



Although many geologists have visited and written about the 

 district, there is no really accurate map of the area covered by this 

 rock ; and the next step found necessary in this investigation was to 

 lay down, as well as the ground would admit, its exact boundaries. 

 At the extreme north-east of the mass a rock was found having a 

 general resemblance to the ordinary Dimetian, but of a more 

 porphyritic nature, the quartzes in particular being bi-pyramidal. 

 A section shows its ground-mass to be granophyric, with a markedly 

 finer structure than anything seen in the normal Dimetian, the 

 other constituents being however identical. It seemed probable 

 that this was only a marginal modification, and this view is 

 supported by the fact that the same rock has been met with again 

 in clear ground at the edge of the mass in several places, especially 

 near Rock House on the north side and near Castell on the north- 

 west side. 



But it is on the west side that the most important evidence has 

 been obtained, where a hitherto unobserved extension of the 

 Dimetian of some size has been met with. Commencing about 

 llhoscribed Farm, a mile south-west of St. David's, this extension 

 can be fairly well traced for a distance of 1100 yards westwards 

 from the main outcrop as hitherto known. It forms a somewhat 

 narrow belt about 300 yards wide, bounded on the north and south 

 by faults, both of which can be seen cutting the Cambrian rocks in 

 the cliff-face of Ramsey Sound. When traced towards the cliffs it 

 was seen to undergo a change in character, and at the western part 

 of the outcrop, about Treginnis-uchaf, is obviously identical with 

 the foliated porphyritic sill already mentioned as occurring at a 



