VOLCANIC GROUP OF ST. DAVID’S. 305 
In the nature then of the rocks which compose the Pre-Cambrian 
mass which has its centre at St. David’s, we have proof ot its 
igneous origin, and of the gradual lessening of the magnitude and 
clearness of the crystals as we approach the boundaries. The 
spherulitic and other structures are also only to be met with in 
truly igneous rocks. This being the case, and the relations of the 
several members recalling those so well described by Prof. Judd in 
the Island of Mull, we are justified in seeing in this group the 
natural products of a volcanic area dissected for us by the hand of 
time. 
But however strong may be the argument derived from this 
particular mass, its force is greatly strengthened by the study of 
two outlying areas, the one on the west and the other on the east, 
in both which the association of the rocks and their minute structure 
suggest, and are explicable by, an igneous origin alone. 
That on the west is the remoter part of Ramsey Island, which 
from a geological point of view presents truly magnificent features. 
The finest section showing the relations of the rocks is seen on the 
south side of Porth Hayog, as in the accompanying sketch (fig. 6). 
Fig. 6.— View in Porth Hayog, looking South. 
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Towards the west we have massive rocks of felspar-porphyry, carved 
by the sea into overhanging arches through which it roars and 
foams, and contributing the huge blocks and rounded pebbles of 
beautiful green-spotted rock which strew the shore. In a ravine 
running south this suddenly comes to an end, and a straight fissure 
is filled with a black substance (owing its blackness to the presence 
Q.J.G.8. No. 158. x 
a a 
