846 Prof. Hughes, On the altered rocks of Anglesea. [Feb. 23, 



of the island around and W. of Amlwch : the second on the W. in 

 and stretching away to the N.E. of Holyhead Island : the third S.E. 

 of the central axis as far as Llangefni : and the fourth touching 

 the Menai Straits. 



Now I would first call attention to this point, that the schists of 

 the central mass are quite different from those which cover the 

 four great areas above defined. There are mica schists as well as 

 hornblende schists and gneiss in the central mass. But most of it 

 seems a truly metamorphic rock in which the minerals have been 

 rearranged, and in which the mica was not deposited as mica, not 

 at any rate in the form and condition in which we find it now. 

 The great masses of the so-called schists of Anglesea away from 

 the central axis I take to be hardly metamorphosed at all, but to 

 be simply gnarled and crumpled rocks which consist principally of 

 thin laminated sandstones and shales, and in which the glossy 

 appearance is, in almost all cases, due to a kind of slickensides per- 

 vading the whole mass as the harder sandy layers were crumpled 

 in the yielding shale \ 



If then the schists of Amlwch, Holyhead, Llangefni, and 

 Menai do not belong to the old truly metamorphic series, what is 

 their place ? To work this out the first and most important ques- 

 tion is, what is their relation to the unaltered black slates and shale? 



It is very rarely that a junction between them and the under- 

 lying series can be seen in section, and it is true that where seen 

 nearly in contact in the little creek of Forth y corwg there is a 

 crack which would be in the position of the fault supposed by the 

 Survey to thi'ow the shales against the gnarled series on the sup- 

 position that the gnarled shales were altered Harlech Beds. But 

 it did not appear to me that there was any great displacement 

 there, and moreover it leaves a portion of the gnarled series on the 

 S. side of the principal crack. There is a mass fallen forward from 

 the cliff which gives the section the appearance of being more 

 disturbed by faults than it is. Besides the improbability of there 

 being a great curved fault running so far exactly along the junc- 

 tion of the black slates and gnarled beds, we can hai-dly allow the 

 possibility in this case of such an enormous downthrow as would 

 be necessary to bring the slaty series from above the horizon of the 

 Llanpadrig quartzites and limestones down to a low part of the 

 gnarled series. And there is nowhere else for them to be thrown 

 from, for the Cambrian is seen in section along the flanks of the 

 central axis, that is to say we have Arenig, Tremadoc and (Lingula 

 Fla^s beingf so far unrecosfnized) we have next below the Tremadoc 



• 1 "r> 



the basement Cambrian conglomerates restmg upon the rrecam- 

 brian gneiss. There is nothing that could be metamorphosed into 

 the gnarled schists. So it seems to me better to accept the more 



1 The specimens exhibited in iUustration are in the Woodwardian Museum. 



