78 DR T. J, JEHU ON 
of their immense size, though these latter are becoming gradually destroyed by blasting 
operations. At many places, especially on the St David’s plateau, huge standing 
stones, cromlechs, or other ancient remains are seen, and it is more reasonable to 
believe that the immense blocks used for these purposes were found as boulders near 
at hand than that they were quarried from the parent mass, which often lies at a 
considerable distance away. And this is rendered the more probable as blocks of 
similar size are not infrequently seen dotted over the surface. An examination of the 
stone dykes will show how plentifully boulders of all sizes must at one time have been 
studded over the ground, and what a variety of rocks is represented. Towards the 
western end of the area the boulders are largely found to have been dispersed from 
the igneous rocks on the north coast of the promontory. And everywhere the great 
majority are derived from parent masses found in the district. Boulders of diabase 
may often be seen resting on volcanic or sedimentary rocks, and vice versa, proving 
that there has been some transport. It has already been mentioned that blocks of 
the St David’s Head gabbro are found lying to the south-east at Caerfai Bay, and 
Caerbwdi Bay, and on the cliffs above. This implies that there was a movement of 
ice from a north-westerly direction, and it agrees with the evidence shown by the 
glacial striae which are seen on the coast. EHrratics are met with often in the drift 
and on the shores, but the number which has been noted on the surface is not great. 
A further study of the stone dykes would doubtless bring more to light 
1. Hvratics seen on the surface of the ground.—The detection of those about to 
be mentioned is mostly due to the fact that they had been broken up by the farmers 
through blasting or other agency, so as to expose fresh surfaces. Huoxs states ((eol. 
Mag., 1891, p. 501) that he observed many northern erratics in the St David’s district. 
The granite boulder which he discovered on Porth-lisky farm “before it was broken 
must have been over 7 feet in length and 3 to 4 feet in thickness, and is identical with 
a porphyritic granite exposed in Anglesea.” He found another of picrite which he thus 
describes: ‘‘ The boulder is somewhat rounded ; its longer axis, which lies nearly south- 
east and north-west, measures about a yard. A transverse section is slightly triangular, 
the shorter sides measuring respectively about 16 inches and 22 inches. It lies on the 
promontory forming the east side of Porth-lisky harbour, resting immediately on 
Dimetian rock, surrounded by an uncultivated area overgrown by gorse and heather” 
(Quart. Journ. Geol. Soc., vol. xli. p. 519). It was submitted to Professor Bonney for 
examination, and he states that it is wonderfully like the boulders found at Pen-y- 
Carnisiog, Anglesea, which had been previously described by him (Quart. Journ. Geol. 
Soc., vol. xxxvil., 1881, p. 137). In a later paper (Quart. Journ. Geol. Soc., vol. xli., 
1885, p. 518), Professor Bonney remarks that “the lithological evidence rather favours 
the derivation of the Anglesea boulders from dykes in that island.” A hornblende 
picrite of a somewhat similar character occurs also im situ at Penarfynydd, on the 
south-west coast of Carnarvonshire. So it is probable that the boulder found near St 
David’s has been carried by the agency of ice from Carnarvonshire or Anglesea. 
a 
