But still further and fuller evidence of Silurian volcanism is to be obtained by an examination of the northern coast-line. I have already referred to the elliptical fault which is marked on the Geological Survey map as running from the north-western headland to the eastern coast beyond Amlwch. The necessity for inserting this fault, apart from any actual visible trace of its occurrence, arose when the conclusion was arrived at that the rocks of the extreme north of Anglesey were essentially altered Cambrian strata.[227] For immediately to the south of these rocks black shales, obviously Silurian, were seen to dip to the north--a structure which could only be accounted for by a dislocation letting them down into that position. The same necessity for a fault has of course been felt by all writers who have subsequently treated the northern area as pre-Cambrian. But it is deserving of notice that in the original mapping of the Survey no continuous abrupt hiatus is shown by the line which was afterwards marked as a continuous line of fault. On the contrary, on one of the field-maps in, I believe, Mr. Selwyn's handwriting the remark occurs:--"The gradual passage from the black shale to the upper green gritty slates of Llanfechell is best seen at Bothedd, on road from Llanfaethlu to Llyn-llygeirian."[228] [Footnote 227: I have fully considered the evidence adduced by Dr. Callaway and Professor Blake, and have examined the ground, and can come to no other conclusion than that stated in the text. But see Mr. Blake's remarks, _Geol. Mag._ 1891, p. 483.] [Footnote 228: There is no continuous section now visible at this place, but the two groups of rock can be traced to within a few feet of each other, both inclined as usual in the same direction, and the black shales appearing to pass under the others.] It is no part of my aim to disprove the existence of faults along the line referred to. These may quite well exist; but there is assuredly no one gigantic displacement, such as the theory I am combating would require; while any faults which do occur cannot be greatly different from the others of the district, and do not prevent the true relations of the rocks from being discoverable.