436 MR. EDWARD GREENLY ON THE [Aug. 1902, 
strong resemblance between eis prevalent types of this area and 
much of the material of the Pentraeth! and N ewborough districts. 
Indeed, I can see no difference at all in some cases, either macro- or 
microscopically. 
Now, along the line above mentioned, the schistose rocks of the 
eustern part of the Eastern Region appear to pass, by a perfectly 
gradual transition, into the holocrystalline schists of the portion to 
the west. Excellent sections are to be seen at a point in Mynydd 
Llwydiarth, 400 feet east-south-east of Hafod Leucu, and on the 
shore of the Straits below Cartrefle, the two ends of the line. The 
change of character affects not only the great body of acidic material, 
but the basic rocks contained in it. The rocks of the region, on 
both sides of the line, appear, therefore, to be a metamorphic unit; 
and there is nothing to show that on the east side we have 
structures of later date than on the west. 
From this, consequently, it would follow that the crystallization of 
the whele region, including its most highly altered members, which 
are undoubted holocrystalline schists, is later than the jaspers, and 
than their incorporation in the schistose rocks of the eastern part 
of the area. 
These two lines of argument lead thus to opposite conclusions ; 
and the second is of such a nature as to call for great caution 
in its acceptance, because of the principles involved in it. For my 
own part, I do not think that there is a sufficient preponderance 
of evidence on either side to justify the pronouncement of a con- 
clusion in this paper: and my aim is, rather to put each case, if it 
may be so spoken of, as strongly as possible. 
The alternatives are, briefly, these :— 
If the grits of Pentraeth can be shown to be contemporaneous 
with, or not later than, the jaspers, then the supposed unity of, and 
eradual transition in, the Eastern Region must be in some way 
de ‘ceptive. If the transition really exists, and the schistose complex 
of the region be a metamorphic unit, then the grits of Pentraeth 
must be of later date than the jasper-bearing group with which 
they are associated. 
I hope that other parts of the island, the mapping of which I have 
not yet completed, may afford evidence that will finally decide which 
alternative must be adopted. 
V. Summary. 
It may conduce to lucidity to summarize briefly the principal 
conclusions and results contained in this paper. 
A red jasper, with the fine shaly material called jaspery phyllite, 
is widely distributed in the southern and south-eastern parts of 
Anglesey, occurring in the districts of Newborough, Pentraeth, and 
Beaumaris. 
Its associates are a varied group of rocks, comprising iene 
diabases, and sexgpentines, with grits and shales. They have been 
much modified by powerful earth-movements, which have produced 
1 J. F. Blake, Quart. Journ. Geol. Soc. vol. xliv (1888) p. 509. 
