390 ME. H. H. THOMAS AND PKOF. 0. T. JONES ON THE [Sept. 1912^ 



Augite-diorites of this type have not been met with in the 

 St. David's region, but this occurrence is of interest in view of 

 the great mass of diorilic rocks of supposed pre-Cambrian age^ 

 which occupy the ridge between Johnston and Talbenny in South 

 Pembrokeshire. 



The basic dykes. — The distribution of the basic dykes which 

 have been observed to cut the older rock-groups has been already 

 discussed. 



Usually they are fairly fine-grained rocks of uniform texture and 

 dark grey to dark bluish-green colour. They become still finer 

 towards their edges, for their margins have been chilled against 

 the rocks into which the dykes were intruded. 



Specimens for microscopic examination were collected from a 

 small opening in one of these dykes, to the south of Silver Hill 

 (E7250), and from the roadway at Pont-yr-hafod (E8991). 

 They prove to be completely albitized, and to consist of lath-shaped 

 crystals of albite arranged iu no particular manner, set in a matrix 

 of chlorite and iron-ore. 



The general structure of these rocks may be judged from fig, 2 B 

 (p. 389), which represents the rock exposed at Pont-yr-hafod. 



III. The Cambrian Eocks. 

 (a) Their Succession and Distribution. 



The Cambrian rocks form a much-torn fringe to the pre-Cambrian. 

 massif, although doubtless they were once continuous over the 

 whole district. They occupy the greatest area near the two ends 

 of the elongated mass, the most complete sections beiug found at 

 the north-eastern extremity in the neighbourhood of the Cleddau 

 Yalley. The succession there obtained is confirmed by sections in 

 the south-western area, which extends from Rhindaston to Pointz 

 Castle ; the highest group, however, has not been recognized in 

 that district, except perhaps on the southern flank of Ehindaston 

 Mountain, near the Kite. The smaller areas of Lower Cambrian 

 rocks which occur along the north-western margin of the complex 

 (north of Tre-rhos, north- Avest of Hayscastle, and Trefgarn-Owen) 

 are of the nature of lenticles, faulted in between the rocks of the 

 massif and the younger rocks which surround it. Similar faulted 

 lenticles also occur within the massif, as to the north-east of Tre- 

 rhos and to the south and east of Troed-y-rhiw. 



The succession in the "Welsh Hook and St. Lawrence district is 

 well displayed down the hill from St. Lawrence Church to Welsh 

 Hook Bridge, and thence north-eastwards along the main road 

 through Welsh Hook, or northwards along the eastern and western 

 banks of the Cleddau. 



The highest beds are developed principally in the neighbourhood 

 of Pord, the best sections being found in the railway-cuttings 

 south and north-west of that hamlet. 



