MONIAN SYSTEir OF EOCKS. 537 



Again, we find quartzites in the series coming in again and again as 

 episodes, and these are not essentially different in character from 

 the masses at Holyhead and Eodafon ; but the grey gneiss is never 

 repeated. These considerations are independent of the amount of 

 metamorphism, which is a somewhat dangerous criterion of age ; yet 

 it must be remarked that there is no more beautifully foliated rock, 

 every particle being crystalline, to be found in the whole of Anglesey 

 than some parts of the grey gneiss, and if we can associate with it 

 the rock of Tafarn-y-botel, we see an approach even to the gneiss of 

 the Highlands. On the other hand the quartzites are dirty rocks, 

 where great fragments of quartz, with no pretence at metamorphosis, 

 lie in dusty material more laminated than foliated, and only partially 

 changed to chlorite and quartz. This is due, no doubt, in part, 

 to the nature of the material, since there is more foliation in the 

 overlying chloritic schists ; but this cannot account for the whole. 



Eegarding, then, the grey gneiss as the base, a rock characterized 

 by the presence of all the three minerals of granite, the next suc- 

 ceeding normal rock is the mica-schist of the eastern district, and of 

 Mynydd Ystum in the Lleyn, of which the quartzites of Holyhead 

 and Eodafon are the episodal equivalents. In these there are present 

 only quartz and either mica or its representative chlorite. 



In succession to these come the chloritic schists. In the Island of 

 Holyhead these are clearly laminated and highly foliated, and retain 

 but few original fragments ; but as we pass eastwards and north- 

 wards they change character, the original deposits are irregular, and 

 the fragments more numerous, till finally they attain quite a breccia- 

 or ash-like character. In the continuous succession of the Eastern 

 District, it is hard to find any representatives of the former facies ; 

 but we seem to come at once, after the mica-schists, to the latter less 

 regularly foliated form : under these circumstances, we must look 

 either to thinning-out of deposits, or to the mica-schists representing 

 in part the chloritic schists, as the solution. If we distinguish the 

 latter facies as the " chloritoid," instead of the " chloritic schists," 

 though they are still chloritic, matters become clearer ; we can thus 

 say that in the central districts the chloritic schists are faulted out 

 of sight, except perhaps near Aberffraw, and that the succeeding 

 rocks are the chloritoid schists. These chloritoid schists are likewise 

 represented from Llanflewin to Mynydd Mechell in the Northern 

 District. 



The succeeding rocks are of different character in the different dis- 

 tricts. This I interpret as due to the occurrence or non- occurrence 

 of volcanic eruptions in the particular area. The period at which such 

 eruptions took place, in some instances at least, can only be sug- 

 gested, since proofs appear to be lacking. The earliest perhaps, are 

 the basic eruptions of the Central and Eastern Districts, whose foci 

 may be found at Craig-yr-allor and the neighbourhood, and at Hol- 

 land Arms ; and with these may be associated the unextruded horn- 

 blende-schists of the east and perhaps the serpentines and gabbros 

 of the west. As their clastic equivalents, partly consisting of fine 

 dust transported from the centre, and partly of the agglomerates 



