VOLCANIC GROUP OF ST. DAVID’S, 303 
acicular microliths. This latter feature becomes predominant in 
the rock of Ogof Llesugn, and is its chief distinction from that at 
Bryn-y-garn. These characters may point to differences in the 
circumstances of formation, but do not indicate any difference in 
material which should satisfactorily characterize a bedded rock. The 
essentially crystalline character of the rock justifies us, under these 
circumstances, in considering it, if not a granite, at least a rock 
formed under similar circumstances, 7. ¢. an essentially igneous rock, 
whose crystals have been formed from a fluid magma. The earlier 
crystallization of the quartz, indicated by the felspar occupying 
sinuous interstices, may be accounted for on well-known chemical 
principles by an excess of the silica in the magma. 
Next, we come to the felsitic rocks. The chief stratigraphical 
feature of these is the complete manner in which they surround the 
central mass, which, for convenience at least, we may designate, 
for the present, as granite. There are two principal varieties, 
which may be called the spotted felsite and the quartz-felsite, 
surrounding the St. David’s granite ; but other forms occur in out- 
lying districts. Commencing at the city, these are found, as is well 
known, one in the Church-School quarry and the other in the Board- 
School quarry, or, rather, both in the first; they are also associated 
in the field to the south, and again in the street; near the cross 
there is the quartz-felsite, and further down the hill, to the west, by 
the Deanery, and on the road-side the spotted felsite, and, at Rock 
House, a more doubtful rock. The spotted felsite continues all 
along the southern side of the Alan from the bridge on the Tre- 
ginnis road, to the bend in the river, when it crosses to the now 
western side and lies between the ashes and the granite. At the 
quarry near Rhoscribed, a piece of the quartz-felsite is let down 
between the granite and conglomerate, and fragments of the spotted 
felsite strew the ground in the field above. The character of the 
rocks on the eastern side is not seen on the road to Caerfai; but on 
the south side again we have the spotted felsite below Nun’s Chapel 
and also further west at Porthclais farm, where the fault cuts out 
everything between the granite and the slate. That the Porthlisky 
tongue is not surrounded by such felsite is accounted for by the 
fact of its being bounded by faults (as I have shown at Ogof 
Llesugn, and as I have observed at Porthlisky Bay, where also 
there is an intrusive dyke of diabase along the line of junction). 
These relations are difficult to explain on the theory of the felsites 
being metamorphosed stratified rocks having any definite strike, 
and are more naturally the result of their being the bounding rocks 
of the granite. Nowhere have I seen any evidence either of the 
granite intruding into them or of their intruding into the granite, 
though neither of these phenomena would surprise me; but they 
intrude into the ashes below Nun’s Chapel. | 
In intimate structure they are essentially igneous rocks, as has 
been practically admitted by ali. The most interesting feature of 
the quartz-felsites is, that while the quartz centres are uniformly 
crystalline, and have therefore crystallized slowly, their boundaries 
