324 



A. GEIKXE ON THE SUPPOSED 



Some of the dykes or veins are only three inches broad. They 

 send out fingers, and sometimes break abruptly across from one line 

 to another. They appear generally to have followed the lines of 

 joint in the granite, as Mr. Tawney has observed*; consequently 

 they must be posterior to the development of the system of jointing 

 in that rock. In many places, particularly in the quarries in the 

 Allan valley, between St. David's and Porth-clais, there is evidence 

 of great pressure having been exercised on the rocks subsequent to 

 the intrusion of the dykes; for the latter are much jointed and 

 slickensided, and even a rude kind of cleavage may occasionally be 

 observed in them. 



Besides the abundant dykes, there has been a more limited extru- 

 sion of the same material in sheets parallel (or approximately so) 

 to the bedding of the sandstones and shales. These sheets are well 

 displayed at St. John's Point, where evidence of their being intrusive, 

 and not truly bedded, may be seen along the fine cliffs which have 

 been cut by the waves on this part of the coast-line. 



6. Conclusion. 



In concluding these observations, I may present a brief summary 

 of what appears to me to have been the geological history of the St. 

 David's district. 



At some remote epoch in the Lower Cambrian period active vol- 

 canic vents, probably submarine, existed in the west of Pembroke- 

 shire. From these vents successive showers of volcanic detritus 

 and occasional streams of lava were emitted, until a pile of volcanic 

 material at least 1800 feet thick had accumulated, Most of the 

 discharges of dust and stones were due to the disruption of basic 

 lavas ; but at successive intervals copious showers of f elsitic debris 

 were also erupted. All the lavas poured out at the surface appear 

 to have been of a basic character (olivine diabase). As volcanic 

 activity died out, ordinary sedimentation was resumed, and the rest 

 of the Harlech and succeeding groups of the Cambrian system were 

 deposited. 



At a later period the whole of these rock-groups, which had been 

 laid down continuously without discordance, were subjected to dis- 

 turbance, the principal effect of which was to throw them into an 

 arch, and to bend over this arch into an isocline, with a general 

 inclination towards the north-west. The strata likewise underwent 

 a wide-spread foliation, which, in accordance with the structure 

 and composition of the rocks affected, was chiefly developed in certain 

 kinds of material. 



Subsequent to these changes the south-eastern side of the fold 

 was invaded by the rise of a mass of granite with the usual peri- 

 pheral quartz porphyries. Accompanying and outlasting this intru- 

 sion, a process of metamorphism went on, the effect of which has 

 been to change fine f elsitic tuff's or shales into hard flinty translu - 

 cent masses, and to superinduce in them a finely crystalline structure 

 * Proc. Nat. Hist. Soc. Bristol, vol. ii. pt. 2 5l p. 115 (1879) 



