454 MR. EDWARD GREENLY ON THE [ Aug. 1902, 
On neither of these points is the evidence conclusive; but on the 
first, thongh imperfect, it is not contradictory, and I will therefore 
discuss this part of the subject before approaching the more 
complicated matter of the second. 
(1) Relation to fossiliferous rocks.—The analogies with 
the rocks of the South of Scotland naturally suggest that the jasper- 
bearing group of Anglesey is, like the Scottish cherts, of Arenig age. 
And it is true that Ordovician rocks do occur in juxtaposition with 
the group in the Pentraeth area, although in the Beaumaris area 
the jaspers and limestones occur quite away from any fossiliferous 
rocks at all. 
But at Pentraeth the known Ordovician rocks (probably Llandeilo- 
Caradoc) are of a different type, and also in a different condition. 
The Ordovician rocks are here, as all over the South-east of Anglesey 
(including the Didymograptus-patulus Beds at the Straits), a very 
uniform series of black shales and dark grits, and always unaltered, 
often not even cleaved. ‘The jasper-bearing group, on the other 
hand, is extremely varied, cut up into lenticles by powerful earth- 
movements, and to a great extent schistose. The very scenery of 
the two groups is sharply contrasted. The same contrast has lately 
been remarked by Mr. Matley in the Lleyn Peninsula, and there the 
fossiliferous rocks are of Lower Arenig age. 
Further, it may be observed that in the mountainous areas of 
Snowdon and of the Harlech anticline, where the whole Ordovician 
Series is exposed from the Bala Beds downward, even into the 
Cambrian, in numerous sections, this jasper-bearing group has 
never been recorded. 
It will thus be seen that there is no positive evidence to connect 
the jaspers of Anglesey with the Arenig Beds, and some negative 
evidence to disconnect them. Moreover, it must not be forgotten 
that the very association of jaspers and pillowy diabases, which 
furnishes the principal argument of this paper, is by no means 
confined to the Arenig Beds, but occurs at several different and 
widely separated horizons. We may therefore expect to find cases 
of this association in rocks of any period. 
(2) Relation to the crystalline schists.—This is the most 
difficult part of the whole subject, for it is here that the conflict of 
evidence to which I have referred comes in. 
The schists which are here meant are those of the region called 
by the Rev. J. F, Blake ‘The Eastern Region’ of the island, 
extending from the coast south of Newborough to the neighbour- 
hood of Beaumaris and Llanddona. That part of this tract which 
lies to the east of a line running from the shore of the Straits a 
little west of the Menai Bridge to Coch y Mieri in Mynydd 
Llywdiarth, is composed of schists which are for the most part but 
minutely crystalline, and contain many lenticles of original clastic 
' Geol. Mag. 1902, p. 122. 
