Vol. 51.] THE COUNTRY AROUND FISHGUARD. 171 



nation applies to the micropoikilitic rocks in this area. The balance 

 of evidence appears to me to be in favour of its applicability. 1 



The microcrystalline condition of the groundmass of some 

 felsites [285], [205], [206], [344] is another type of alteration. 

 In some cases the porphyritic felspars are obliterated ; in others they 

 remain distinct, at least in the early stages of the development of 

 the microcrystalline aggregate [206]. The boundaries of the in- 

 dividual grains are fairly distinct. 



Slight local variations of tbe groundmass, probably due to original 

 differences in texture and composition influencing the mode of 

 devitrification and secondary replacement, are noticeable in a few 

 cases. Thus a rock from Trefelgarn [217] has parts of the ground- 

 mass microlitic, and other parts microcrystalline and spberulitic. 



In a very vesicular rock from Cnwc Mawr [305], we meet with a 

 spherulitic type of devitrification in the groundmass. The spheru- 

 lites show an indistinct black cross with crossed nicols, but bave no 

 definite outlines and merge insensibly into the rest of the ground- 

 mass. Some have been formed round a quartz-grain at the centre. 

 ' Pseudo-spherulites ' would be, I think, a better designation than 

 spherulites proper. In other cases, where the microcrystalline con- 

 dition is strongly and most completely developed, it is difficult to 

 know the original nature of the rock or to decide whether it is of 

 pyroclastic origin or not ; take, for instance [285]. 



There are a few rocks from the tract just north of Cam Ffoi, near 

 Newport, which, though undoubtedly felsites, show some uncommon 

 features. "When viewed under the microscope they [315], [334], 

 [335], [340] all at first sight have the appearance of vesicular 

 rhyolites. The groundmass may be either cryptocrystalline, micro- 

 crystalline, or micropoikilitic, and varies considerably in each section. 

 There are a number of clear spots scattered about in it ; some are 

 circular, others are oval, others are irregular in shape, but nearly 

 all are bordered by a line of dark green granules which form a 

 definite encircling ring. The clear central portion sometimes shows 

 radial structure, and gives a more or less distinct black cross with 

 crossed nicols ; but in other cases the interior is cryptocrystalline, 

 or behaves like a patch of the blurred mosaic in micropoikilitic 

 felsites. 



The green granules that surround the clear spots may be also 

 seen scattered sparingly through the rest of the rock, in addition to 

 grains, flecks, and shreds of the same colour. In some cases the 

 green ring round a spot is broken and irregular, or it may be 

 entirely absent. But when present it is invariably narrow, and 

 looks merely like a green line. Large porphyritic felspars with low 

 extinction-augles occur in some of the sections : [315], [340]. 



1 The foregoing conclusions, which were written out some months before the 

 appearance of Mr. Harker's first paper on Carrock Fell (Quart. Journ. Greol. 

 Soc. vol. 1. 1894, p. 311), are supported by his observations on the meta- 

 inorphism of the surrounding lavas and the development of micropoikilitic 

 structure by the intrusive gabbro. 



