RECENTLY EXPOSED NEAR BEAUMABIS. 475 



microscopically the similarity is less marked. These Anglesey spe- 

 cimens correspond most nearly with the slides obtained from various 

 parts of the series which extends from the coarse breccia at Tair- 

 ffynnon Quarry up to near the horizon of the Cae Seri rock. 



The argillite associated with these grits does not resemble any of 

 the specimens which I have examined on the other side of the 

 Straits ; but there I chiefly directed my attention to the coarser 

 beds as more important for classificatory purposes, so that I lay no 

 stress on this difference. Under the microscope it appears, at first 

 glance, not unlike a volcanic glass, consisting of a fairly clear mate- 

 rial irregularly tinged with extremely minute granules of ferrite 

 with a slightly wavy banding. In this are scattered rather angular 

 crystalline particles and some patches of a dusky granular mineral. 

 With crossed nicols the ground-mass remains black, except for the 

 presence of many minute crystallites of variable form and often frag- 

 mental aspect. The exact nature of these I cannot determine : 

 some look like quartz, others may be felspar or more probably a 

 zeolitic mineral; the larger patches are granular in structure, in 

 some cases concretionary, in others resembling pseudomorphs after 

 a crystal of felspar. With ordinary light they are a pale dull 

 grey, like the colour of very diluted ink ; with polarized light they 

 prove to be very dichroic, becoming almost black in one position ; 

 with crossed nicols they change from a sort of buff tint to black ; 

 and on selecting one of the most definite forms, I find that extinction 

 takes place when its longer axis makes an angle of 45° with the 

 vibration-plane of the nicols. I am unable to identify this mineral. 



The same mineral occurs occasionally in some of the volcanic 

 fragments in the other slides. I should regard a volcanic mud as 

 the most probable material of this rock. It does not at all resemble 

 the ordinary argillites or porcellanites, and, if a little more altered, 

 would, I think, be a very typical halleflinta, i. e. a rock of sedimen- 

 tary origin by no means easy to distinguish from a compact felsite. 



The relations of these two groups of rocks are, I think, of impor- 

 tance in reference to some matters of rather recent controversy. 

 Their junction is most probably a faulted one, and no one accus- 

 tomed to the study of rocks can doubt that the felsitic-grit series is 

 much newer than the schists. The former rocks are but little 

 metamorphosed ; the alteration in them is not greater than we find 

 in such beds as those at or just below the base of the Cambrian in 

 Caernarvonshire or Pembrokeshire, at Charnwood, or at the Wrekin ; 

 the latter have undergone very considerable metamorphism. As a 

 rule, no original constituent in them can be identified with certainty. 

 I have examined several specimens from this great series of schists, 

 which extends from the west of the Menai Bridge to the neighbour- 

 hood of Beaumaris, and only in the case of the quartzite and schist 

 of Pen-y-Parc have I detected original constituents. Once, doubt- 

 less, mudstones of variable chemical composition, they are now true 

 foliated rocks, their chemical constituents having entered into new 

 mineral combinations. Changes of this kind may doubtless some- 

 times occur under exceptional circumstances in a series generally 



