380 EEPORT— 1888. 



bands of dustiei" material are seen to pass in an undulating manner across 

 the slide, marking the contortions to which the rock has been subjected. 

 But the crystals of green mica have no relation to these lines of bedding, 

 but are arranged somewhat obliquely with reference to a series of parallel 

 cracks, which have produced what is practically a strain-slip cleavage. 

 The direction, therefore, of the orientation is governed by the stresses to 

 which the rocks have been subjected. A somewhat similar phenomenon 

 is seen in a rock at Bodedern (23), only in addition the mica is so 

 crowded in certain bands as to be satiny, and the orientation becomes iu 

 part elemental. 



An increase in the green mica of such rocks produces very beautiful 

 and characteristic results. In some cases the fine flakes are so crowded, 

 and have an orientation in two directions, that they produce a kind of 

 felted structure (see fig. 11). Such a kind of rock is seen below Bodafon 

 Mountain (110), where there is least regularity in the flakes. In the so- 

 called Silurian at Llanfaelog (82), and at Bodlew in the Eastern district 

 (147), one set of flakes pass in one direction, and a smaller set are at 

 right angles to them. But at Trewjn, near Bodafon (104), and at Bo- 

 dewryd, in the Northern district (220), there is a very close quincuncial 

 orientation, with a laminar orientation of ferruginous dust. These differ- 

 ences doubtless depend on the original material of the rock ; but, prac- 

 tically, the metamorphism of these felted rocks and that of the mosaic 

 rocks has gone as far as it can in either case. 



We now pass to the chemical alterations eS'ected in rocks of larger 

 elements. Some of these we have already seen to have their matrix pass 

 over into a mosaic, with or without the addition of some green-mica. 

 Others, in which the large elements are more abundant, as that to the 

 west of Llanfechell (219) and near Llanrhwydrus (213), have simply chlo- 

 rite infused between the fragments ; a method of alteration which attains 

 its maximum at Bryn Alinceg, near Llandegfan (189). In other cases, as 

 at Ty Croes (85), green mica is the interstitial mineral ; and in others, 

 as near Bodafon Farm (106), both are present, particularly in the neigh- 

 bourhood of some parallel cracks which give a bedded appearance to 

 the rock. 



More interesting is the study of the quartzitic rocks, in which altera- 

 tion would seem harder to induce, and less easy to be made to result in 

 orientation. Tet the greater number of those examined are known to 

 show cleavage on a large scale. Of the ten rocks of this kind examined, 

 three belong to the Bodafon district, two to Holyhead, and five to the 

 South Stack Series. Now, it is known that while the rocks of the two 

 latter localities cleave well, those of the first do not. This difference in 

 behaviour ought to correspond to some difference in structure. A very 

 short examination shows there is a marked difference. In the three 

 Bodafon rocks from Carnedd a Tre'r beidr (107), the summit of the 

 hill (109), and Craig Fryr (110), the orientation is elemental ; there is 

 abundant green mica or sericite, but the quartz fragments are themselves 

 elongated, and their junctions have become sutural where they come into 

 contact, and they show further signs of pressure. In contrast to this, 

 the rocks from the other districts have no elemental orientation, and 

 show fewer signs of pressure. The deduction from these facts appears 

 to be that elemental orientation is not favourable io cleavage, and that 

 though cleavage may be produced by pressure, there is a point beyond 

 which, if the pressure be increased, the cleavage is destroyed again. 



