Vol. 58. ] JASPERS OF SOUTH-EASTERN ANGLESEY, 435 
matter |; whereas the part to the west of this line consists of holo- 
crystalline mica-schist in which no such original structures can be 
discerned. ‘To discuss the origin of these rocks is no part of the 
purpose of this paper, and would open up large and far-reaching 
questions. I shall therefore describe their phenomena only so far as 
is necessary to a discussion of their relation to the jaspers. 
There are two chains of evidence, and they had better be con- 
sidered separately. 
a.—The jasper-bearing group at Pentraeth and Newborough, 
either adjoins the crystalline schists of the western and most com- 
pletely altered area, or is separated from them only by a narrow 
belt of Ordovician shales ; and they are very different in condition. 
Nowhere in the Jjasper-bearing group in these areas, however 
highly sheared and altered they may be, are there any rocks the 
minerals of which indicate a very high temperature or very deep- 
seated conditions, except the micas in some of the schistose material, 
which do indeed appear to be authigenetic. The basic igneous rocks 
never pass beyond the stage of chloritic and epidotic schists: no 
hornblende has been observed in them ; whereas the basic rocks of 
the adjacent complex are always true hornblende-schists, sometimes 
even hornblendic gneisses. Certainly it would seem extremely un- 
likely that there could be any connection between rocks at once so 
near together and so different in crystalline condition. 
Moreover, a slide of one of the ashy grits of the Pentraeth area 
contains two fragments of a thoroughly crystalline mica-schist of 
the type most prevalent in the adjacent complex. The condition of 
the grit, and the mode of occurrence of the fragments, put a cata- 
clastic origin out of the question. This grit must, therefore, be later 
than the crystallization of the mica-schists. It is associated with 
the jasper-bearing group, and if contemporaneous, that group 
must also be later than the crystallization of the schists in 
question. 
b.—The second chain of evidence is as follows. The jaspers and 
limestones are found not only in the Pentraeth and Newborough areas, 
but also in the eastern part of the Eastern Schistose Region, that 
is, east of the line from the Menai Bridge to Mynydd Llwydiarth. 
They occur there as lenticles in the schists, and in such a way as 
to make it almost incredible that the structures of the enveloping 
rocks have not been developed since the jaspers and limestones 
became incorporated in them.” In Baron-Hill woods, lenticles of 
jasper, not more than 3 to 5 millimetres thick, are wrapped round 
by the folia of the fine schistose rocks exactly as any other phacoidal 
masses in them are. At Crymlyn, in the heart of the plateau, a 
jaspery limestone occurs in the schistose rocks, and a ravine some 
30 feet deep has been cut through it and them. There is a very 
* KE. Greenly, Geol. Mag. 1896, p. 551. 
* This would still be true, even if the rocks of this region were, as they very 
possibly may be, a complex of immaterial of different ages. 
