MONIAN SYSTEM OF ROCKS. 



507 



from the rest. Yet in none of these, except at Tros-y-gors, is any 

 regular foliation found. The plates lie any way, and are composed 

 of the finest crystalline flakes. Beyond this line are various rocks, 

 some intimately crystalline, such as those at Clawd-y-parc, north 

 of Llandegfan, and at Coed Cadw, but the majority of those 

 examined, as types of the rest, are either full of angular fragments 

 or consist of fine unaltered dust. Of the first kind are the rock at 

 Pen-y-parc, described by Prof. Bonney, that at Ty-gwyn south of 

 Llandegfan, at Bryn Minceg, about a mile to the north, and at Coed 

 Mawr, near Leanfaes ; of the latter are the rocks at Ty Garw, near 

 Pen-y-parc, at Llyn Bodgolched, and at Tyddyn, north of Beaumaris. 

 As it is impossible to tell the structure of a rock without looking at 

 it microscopically, I cannot say how far these are really typical of 

 the rest, but only that the whole much more resembles the upper 

 part of the series in both the Western and Central Districts than the 

 lower or even than the fine-grained chloritic schists of the mainland. 

 The characteristic features, however, sometimes come out far better 

 in viewing the rock on a large scale than in a hand-specimen or 

 a slide. Thus on the road between Garth Ferry and Beaumaris 

 the roadside cliffs show most beautifully the arrangement of the 

 materials, and it is certainly not that of an ordinary stratified rock, 

 however contorted (see fig. 15). I take the twisted lenticles of 



Fig. 15. — Weathered surface of Rock, north of Garth Ferry. 



1. Quartz. 



2. Schist. 



quartz to be authigenetic, and caused by a kind of quivering, under 

 pressure, of an ill-compacted, practically tuf aceous rock. A kind of 

 succession may also here be traced, for after this comes some compact 

 rock, like the marbled slate, then some slaty bands dipping north- 

 wards and without cleavage. After this, in the road-cutting above, 

 before referred to, are some beds with large quartz-lumps, then 

 some disturbed rocks with much chlorite, such as is seen also at 

 Gallow's Point, and, finally some lenticular polite. No such varia- 

 tions as these are seen in the lower part of the series, and though 

 agglomerates are apparently wanting here, we can still look to 

 eruptive sources as the most probable origin for the material. 

 Similar instructive varieties, of a slaty or marbled kind, with calca- 



