243 



EEV. J. F. BLAKE ON THE ROCKS MAPPED AS 



beds have been let down to a lower level by the fault marked on the 

 Geological Survey map. The same is well seen also where the 

 lowest grit bed is exposed on Bronllwyd itself, just at the turn of 

 the highest slate-railway. Here the lumps of included slate are 

 darker than usual, but they are still angular. The same breccia 

 is also seen on the slopes of Ffridd-y-Fedw in the extreme north, 

 and is there an admirable guide to the junction-line. Nowhere 

 along the line that I have shown to be a fault has this breccia 

 been met with. 



As to whether the Bronllwyd Grit lies upon different portions of 

 the Purple Slate series or not is by no means clear. There are 

 signs that it does overlap, which might be accepted as corroborative 

 evidence, bat which alone could not be expected to carry conviction 

 to a doubter. Thus, at Braich Melyn and Bronllwyd the bed imme- 

 diately below is the Pale Slate, but at Glan-y-gors the grit lies 

 directly on Purple Slate. This latter is called the ' slate of the Tram- 

 way cutting ' by Prof. Hughes {op. cit.), and is placed by him below 

 the whole of the Penrhyn Quarry slates. If this determination of 

 their position were correct, it would prove an unconformity, but a 

 fault, of which there is ample evidence, is inserted on the Survey map 

 between this spot and the Penrhyn Quarries. Farther north the 

 rocks, as will be seen by. the map, run in a circle, and at different 

 distances along the radii in different directions are found varieties 

 of slate which we may fancy we recognize as corresponding in due 

 order to those of the Penrhyn Quarries — and in particular the expo- 

 sures at the greatest distance are of Pale Slate. Yet the overlying 

 grit rests indiscriminately on these various members. There is 

 nothing to show, however, that these varieties are constant in posi- 

 tion and may not come on in different localities on the same horizon. 

 In any case the grit is never seen to lie on anything but some part 

 of the Upper Purple Slates, so that the unconformity, if existing, is 

 not demonstrated to be a large one in this locality. 



In studying the distribution of the remaining rocks referred to 

 the Cambrian, it was natural to take in the first instance the Geolo- 

 gical Survey map as a guide. On this map numerous bands of grit 

 are marked and their distribution traced, but though in the sections 

 they are numbered they are not distinguished in mapping, and 

 numerous faults are inserted, so that the map is exceedingly compli- 

 cated. Indeed, I have found it unintelligible on the ground. With 

 some of the main features my own observations agree, but after a 

 sincere attempt to verify the details, and the demonstration therein 

 that as tested by the six-inch map many of them were indubitably 

 erroneous, I have had at last to commence with a tabula rasa, and the 

 result is that my map in almost all its minor details differs from that 

 of the Geological Survey (see PI. VI.). I do not for a moment suppose 

 that my map is absolutely correct ; but I have examined the rocks 

 at every discoverable exposure in the whole area, though I may have 

 wrongly identified some of them. So far as the observations are 

 correct, the map is drawn so as to be consistent with them all — but 



