156 
THE CAMBRIAN VOLCANOES 
BOOK in 
times assumes a more decidedly pegmatitic structiu'e, like graphic granite. 
At the northern end of the granite ridge, a gradation can he traced from the 
ordinary textirre through increasingly fine-grained varieties until we pass 
into a microcrystalline spherulitic porphyry. After a careful examination 
of the ground I satisfied myself that the spherulitic quartz-porphyries, which 
form a conspicuous feature in the geology of St. David’s, are really offshoots 
from this granitic core.’ 
These spherulitic rocks have been fully described.' They consist of a 
base composed of a inicrocrystalliue aggregate of quartz and orthoclase, 
which is distributed between the spherulites. These have been developed in 
remarkable beauty and perfection. While the microcrystalline structure is 
everywhere recognizable, the spherulites occasionally disappear. But their 
absence is merely local, and they may be foTind both in large dykes and 
narrow veins. A further porphyritic structure is given to the rocks by the 
presence in them of abundant quartz, which takes the form of conspicuous 
rounded blebs or worn crystals sometimes distinctly dihexihedral, but with 
somewhat blunted angles. Porphyritic plagioclase is also common. Flow- 
structure is occasionally traceable. Some parts of these rocks where the 
porphyritic elements are locally absent might be cursorily classed as felsites ; 
but they all possess a microcrystalliiie and not a felsitic base. They 
cannot be confounded with the true felsites of which fragments occur in 
the tuffs. 
In addition to the parallelism that may be traced between the earliest 
Palaeozoic agglomerates and those of the youngest volcanic series of 
Britain, a close analogy may also be noticed between the acid intrusive rocks 
of the two widely-separated periods. In both cases we have a granitic 
core sending out apophyses which assmne a spherulitic structure and traverse 
earlier volcanic products of the district. 
These spherulitic quartz-porphyries of St. David’s occur as bosses, dykes 
(elvans) or veins, cutting through all horizons of the volcanic group, and in 
one case apparently, if not actually, reaching the quartz conglomerate. One 
of the best exposures of this intrusive character may be seen in the cliff 
below Kim’s Chapel, where the elvan runs along the face of the cliff through 
the uppermost zone of the volcanic group, cutting the strata somewhat 
irregularly. Apparently in connection with this dyke, a network of 
intrusions of decomposed quartz-porphyry may be observed in the shales 
along the face of the cliff immediately below Kun’s Chapel. On the 
whole, the intruded material has forced its way along the bedding-planes 
of the shales, but has also broken across them, sending out finger -like 
1 iranches. 
3. Diabase, Dykes and Sills . — The latest rocks of the St. David’s district 
are dykes and intrusive sheets of diabase, which traverse all the other 
' These apopliyses from the granite constitute the “Arvouian” formation of Dr. Hicks’ 
pre-Cambrian series of St. David’s. 
^ See, for example, J. Davies, Quart. Journ. Geol. Soc. vol. xxiv. p. 164, xxxv. p. 203 ; also 
the paper already referred to, op. cit. xxxix. p. 315 ; and Mr. Teall’s British Petrography, p. 334. 
