Vol. 58. ] JASPERS OF SOUTH-EASTERN ANGLESEY. 437 
brecciated and schistose structures through large parts of the areas, 
but original structures have survived in many places, and here the 
true relations of the rocks can often be seen. 
The associations of the jaspers with the limestones, and especially 
with the diabases, are of the most intimate nature. 
The diabases have the same characters, both in the field and 
under the microscope, as those basic lavas possessing the so-called 
‘pillowy’ structure (often also variolitic), which have been found 
to be associated with radiolarian cherts and jaspers in several 
different parts of the world, and at several different geological 
horizons. Moreover, the relation of the jaspers to the igneous 
rocks here, is the same as that of the radiolarian chert in the South 
of Scotland to the accompanying igneous rocks. 
On this ground, and also from the occurrence of jaspery phyllite 
with evident clastic texture, it is inferred that the jaspers of 
Anglesey are of organic origin, and must be regarded as altered 
radiolarian cherts. 
The evidence for the age of the group is much less satisfactory. 
There is not sufficient to refer it to the Arenic Series, and it is 
possible that it belongs to a different period altogether. 
Its relation to the crystalline schists of the region is obscured by 
conflicting evidence, which cannot yet be reconciled: one chain of 
evidence leading to the view that the group is older than, and has 
been involved in, the metamorphism of the adjacent schists, while 
another gives strong reason to suppose that it is altogether later. 
EXPLANATION OF PLATES XV & XVI. 
Puate XV. 
Fig. 1 [No. 6, 1899] Jasper, from 200 yards west of Bryn Hyfryd, Llansadwrn. 
The minerals shown are quartz, hematite, and a carbonate. Most of 
the hematite is in fine dust, but there are many flakes of definite 
eisenglimmer. The aggregates of hematite-dust which produce the 
mottled appearance are contained within single optical individuals of 
the quartz-mosaic. In the middle of the field is a rhombic section of a | 
carbonate, with a zone of opaque hematite and a crystal of eisenglimmer. 
The light parts are quartz-veins. (x 36.) 
2[914] Jaspery phyllite, from Bryn Mawr, south-south-west of 
Pentraeth.—The rock consists of a remarkably transparent matrix 
full of hzmatite-dust. ‘The small clastic grains seen are quartz and 
white mica. Thin quartz-veins cross the field. (x 36.) 
PuatE XVI. 
Fig. 1 [55a] Diabase, from Fferam-gorniog, Pentraeth.—The only minerals 
shown are felspar and hematite-dust. The felspars have a sheaf-like 
or imperfectly radial arrangement. This rock contains small oval 
amygdules. (xX 36.) 
2 [No. 5, 1899] Variolite, from Taw y Graig, south-west of Pentraeth.— 
The only well-defined mineral is a felspar. The groundmass, though 
green with alteration-products, remains almost dark between crossed 
nicols. (X 36.) 
| Fig. 1 in Pl. XV and fig. 2in Pl. XVI are reproduced from slides kindly lent 
by Mr. George Barrow. He had them cut from specimens collected on the 
occasion of his visiting the ground with me, and they are now in his private 
collection. Fig. 2 in Pl. XV and fig. 1 in Pl. XVI are from slides in my own 
collection. | 
Q.J.G.8. No. 281. 25 
