482 PROF. W. S. BOTJLTON ON THE [NOV. I904, 



a relatively-fresh and unaltered portion of the rock, whereas, owing 

 to the absence of quarries or other deep openings, those from 

 Pontesford were taken from near the surface. The general resem- 

 blance in chemical composition, mineral contents, and structures of 

 the Pontesford rock and that of Rowley would apply also to the 

 dolerites of the Clee Hills, Kinlet, and the east of the Wrekin, 

 and applies almost equally well to some of the Tertiary dolerites 

 of Scotland, described by Prof. Judd. 1 On the other hand, it 

 differs from most of the intrusive dolerites of North Wales, mainly 

 in the absence of olivine in these latter rocks ; while the olivine- 

 diabase or basalt-lava, associated with the Pebidian rocks of 

 St. Davids (Pembrokeshire), described by Sir Archibald Geikie, 2 

 differs from the Pontesford rock, mainly in the much greater amount 

 of magnesia and smaller quantity of potash in the Pembrokeshire 

 rock. 



(8) Relation of the Intrusive Basic Rocks to the 

 Bedded Rocks. 



Although the general trend of the dolerite-masses of the hill is 

 with the bedding of the tuffs, there can be no doubt that the 

 dolerite is intrusive in the latter, notwithstanding that some of the 

 characters, such as the coarsely-vesicular structure and the parallel 

 arrangement of vesicles and felspar-crystals, are those generally 

 associated with bedded lava-flows. In some places (516 <-?, 201) 

 the dolerite can be seen penetrating, and enclosing masses of, the 

 andesite-tuffs and lava, while the irregular junction of the dolerite 

 and the bedded rocks (see map, PI. XXXYIII) would preclude the 

 possibility of its being interbedded with the latter. 



It is highly probable that the dolerite forced its way into the 

 underlying tuff's and lavas, mainly along two lines of weakness, 

 near the south-eastern margins of the two dolerite-masses shown on 

 the map, and, with relief of pressure, spread out among the bedded 

 rocks. But whether the dolerite actuallv came out at the surface, 

 or formed a laccolitic mass between the tuff-series and newer beds 

 now removed (possibly Cambrian or Bala, both of which are in the 

 immediate vicinity), there is no direct evidence to show. It would 

 seem more probable, however, that the dolerite invaded the bedded 

 rocks during the disturbance of the latter, which resulted in their 

 present high inclination (see map and sections, Pis. XXXVIII & 

 XXXIX). 



IY. General Summart of Conclusions. 



The present paper is confined to a description of the characters 

 and sequence of the rocks within the limits of Pontesford Hill, and 

 no attempt is here made to correlate them with those of other 

 Uriconian areas. 



1 Quart. Journ. Geol. Soc. vol. xlii (1886). 



2 Ibid. vol. xxxix (1883) p. 293. 



