CHAP. X 
Sr. DAVID’S 
155 
of fine I'eddish tuff (os) are succeeded by a band of ij^uartz conglomerate 
(6) of the usual character. Parallel to this conglomerate comes a hand, 
about six inches thick, of fine tuff (c), followed by ashy sandstone (c?), which 
graduates into conglomerate (e). No more complete evidence could be 
desired of the perfect inosculation of the conglomerate with the volcanic 
group. On the coast at Nun’s Chapel similar evidence presents itself. The 
conglomerate there contains some thin seams of tuff, and is intercalated 
between a series of dull green agglomerates and tuffs and some greenish 
shales and sandstones with layers of tuff. 
Not less striking is the evidence of the contemporaneoits eruption of fine 
volcanic dust in the overlying shales and sandstones.^ Some of the red 
shales are full of this material, which here and there is gathered into the 
thin seams or ribs of which the microscopic characters have already been 
described. I’his diffused volcanic detritus marks, no doubt, the enteebled 
discharges of fine dust towards the close of the volcanic episode in the 
Lower Cambrian period at St. David’s. It would be difficult to find an 
instance of a more perfect transition from a series of thoroughly volcanic 
masses into a series of ordinary mechanical sediments. 
2. Intrusive Granite and Quartz-Forphyry . — The heart of the volcanic 
group is pierced by a mass of granite which also cuts the conglomerate and 
overlying shales and sandstones on the east side. Tlie age of this intrusive 
boss cannot be more definitely fixed than by saying that it must be later 
than the volcanic group. This rock has been the subject of a remarkable 
amount of description, and has been dignified by being actually elevated info 
a distinct Archaean “ formation ” composed of “ highly crystalline gneissic 
rocks,” with “ bands of limestoire, horir blende, chlorite, and micaceous 
schists.”^ Into this somewhat dreary chapter of English geological litera- 
ture it is fortunately not necessary to enter here. I will oirly say that the 
rock is unquestionably a granite, with no essential differences from many 
other bosses regarding which there has beeir no controversy. It is a holo- 
crystalline rock witli a thoroughly granitic texture, and composed of the 
ordinary minerals of granite — quartz, orthoclase and plagioclase, among 
which a green chloritic mineral, more or less plentiful, probably represents 
original hornblende, biotite or augite. Sometimes the quartz and felspar in 
the body of the rock show a inicropegmatitic arrangement, and the same 
structure occurs in \eins that traverse it. This structure gives the rock 
some resemblance to the Tertiary granites and granophyres of Ireland and 
Scotland. 
This granite has undergone a good deal of decomposition, for its felspars 
are turbid, and its original ferro - magnesian constituents are always 
replaced by green chloritic aggregates, while epidote is also present. The 
rock tends to become finer in grain towards the margin, and then some- 
^ These are a portion of Dr. Hicks’ “Caerfai group” in the Lower Cambrian series. They 
liave yielded Lower Cambrian fossils. _ ■ ■ n t 
^ See the papers cited on p. 145 and niy discussion of the relations of this granite in Quart. 
J'ouni. Geol. Hoc. vol. xx.xix. ; also Prof. Lloyd Morgan, op. cit. vol. xlvi. (1890). 
