PROF. A. H. GREEX OX A SECTION NEAR LLANBERIS. 77 



attention to the strong cleavage and the flattening of the fragments 

 in the rocks B and C, and he shows these rocks as vertical on the 

 south-east, so far agreeing with myself : but on the north-west he 

 makes the beds flatten, and draws across the flaky rocks lines of 

 bedding sloping at a moderate angle to the north-west, running in 

 fact parallel to the junction of the conglomerate and the flaky rocks. 

 In short he makes no distinction between the flaky breccias and the 

 conglomerate, but looks upon the former as a part* of the latter 

 somewhat altered in character by intense cleavage. It is with no 

 small amount of diffidence and regret that I feel bound to differ 

 from my old honoured master ; but I feel sure that the bedding of 

 the flaky breccias is nearly vertical from one end of the exposure to 

 the other. Further, these breccias prove on examination to diff'er 

 totally from the conglomerate, and, indeed, from any part of the 

 Harlech and Llanberis group, in their character and origin. I 

 cannot therefore accept his interpretation. 



To this point, the nature and origin of the flaky breccias, we 

 next come. Under the microscope, with a power of 40 diameters, 

 the matrix of C is seen to be fine dust full of minute plates and 

 films of a micaceous mineral that, under crossed nicols, is golden 

 yellow shot with green and red. Among the fragments are a few 

 blebs of quartz, bits of kaolinized felspar, a fragment very like a bit 

 of quartz-felsite, and several pieces of dark vesicular scoriae ; but the 

 larger part of the fragments are not exactly determinable on account 

 of the alteration they have undergone ; they remind one of basic 

 crystalline rocks that have been very thoroughly serpentinized and 

 viriditized, and in part converted into calcite. 



The matrix of B is generally similar to that of C, but finer and 

 more uniform in grain ; it lies in long wavy bands which bend 

 round the fragments and simulate very closely true fluxion-structure; 

 the fragments are most of them similar to those of C, but not so 

 numerous, and quartz is more abundant ; there are a few plates of 

 mica and some bits that remind one of fragments of crystalline 

 schist. 



In general character both these rocks are ver^- like rather coarse 

 volcanic tuffs, and the presence of vesicular scoriae in C is in favour 

 of the view that this is their character. Professor A. Geikie tells 

 me that they are very like some of the tuff's of St. Davids which are 

 classed by Dr. Hicks as Pebidian. 



. It may be desirable, though perhaps hardly necessary, to point 

 out that the unconformity of this section, strong as it is, does not 

 necessarily indicate any great difference in age between the con- 

 glomerate and the breccia on which it lies. These breccias are of 

 volcanic origin, and the irregular and restricted upheavals and dis- 

 turbances, which are always liable to occur where volcanic activity 

 is going on, are quite competent to bring about unconformities 



* "Part of the conglomerate consists of slaty pebbles in a slaty matrix, the 

 ■whole being affected by slaty cleavage, remarkable on acccnnt of the pebbles 

 being elongated in the direction ot the cleaTage-liaes." ''Geol. of Is"". Wales, 

 p. 179.) 



