292 T. DAVIES ON THE MICROSCOPICAL STRUCTURE 
cryptocrystalline base is enclosed in wavy bands, which appear to have 
been disturbed in their parallelism by the subsequent development 
of the microcrystalline nests. These nests and also the fissure-like 
groupings are distributed throughout, and are encircled by bands 
of a fibrous chalcedony, the structure of which is well exhibited with 
polarized light, the foliation or stratification being more markedly 
illustrated than in No. 1. In one part of the section of this rock 
undoubted angular fragments, distinct in size and shape from the 
mass, are enclosed, affording further evidence of its probable original 
sedimentary origin. Similar minute needles and spots of the opaque 
and sometimes translucent brown mineral are present here as in 
No. 1. 
3. Treffgarn Rocks on Fishguard Road (p. 287).—Another rock of 
the same type as the two preceding, its structural peculiarities being 
almost identical with No. 2. The nests of quartz are, however, 
somewhat coarser-grained, while the indications of foliation or stra- 
tification are not so distinct. Exceedingly minute needles are dis- 
tributed throughout, here and there aggregated into dense groups. 
Under a high objective these are resolved into very distinct trans- 
parent crystals, which depolarize ight. I have not yet been able 
to satisfy myself as to their mineral nature, but believe them to 
be related to hornblende. With these are associated numerous 
opaque or semitranslucent crystals, occasionally presenting a hexa- 
gonal section, and which are probably magnetite. 
The macroscopical and microscopical characters of these rocks are 
so remarkably like those of the hilleflintas from Sweden, that they 
are not to be differentiated by means of the microscope. The same 
variations in texture, the indefinite character of the interstitial 
felsitic constituent, the presence of the numerous acicular crystals 
(only seen to be such by the use of high objectives), are also cha- 
racteristic of their Swedish prototypes, to which I propose to refer 
them; and so far as their present structure (as determined by the 
microscope) can indicate their origin, it would appear to have been a 
sedimentary one. 
4. Road south of the Deanery, St. David’s (p. 289).—This is a 
greenish-grey rock with blackish-grey spots, and exceedingly fis- 
sured ; this and its spotted character give it the aspect of a breccia. 
Examined in thin sections it presents a microcrystalline ground-mass 
of quartz and felsitic matter with a thickly distributed undeterminable 
dark grey dust-like substance, much resembling that found in many 
of the so-called “ volcanic ashes,’ mingled with grains of a black 
opaque mineral with the habit of magnetite. The spotted appear- 
ance of the rock is due to the denser aggregation of this substance 
in patches, associated with viridite. Crystals or parts of crystals of 
felspar, mostly orthoclase, are occasionally discernible, while indis- 
tinct columnar crystals of a grey colour, becoming black between 
crossed Nicols, are thickly distributed in parts of the ground-mass. 
5. Road south of Deanery, St. David’s (p. 289).—Apparently the 
same rock as No. 4, but the brecciated aspect is here not apparent. 
A thin section discloses a coarser microcrystalline ground-mass 
