532 EEY. J. p. BLAKE ON THE 



must be faulted up along with the rest, which would require very- 

 little modification of the lines on the map, and those only in places 

 where nothing is seen. Though its association with the rocks of 

 Pre-Cambrian character suggests its age, the only positive evidence 

 adducible is its similarity of structure to the corresponding associates 

 elsewhere. Where it is in its greatest mass, on Mynydd Cefn Amlwch, 

 it is a very quartzose, coarse, granitic rock, with very little mica, 

 thus corresponding to the granite of Forth Lygan and to that of 

 Twt Hill. Further west it becomes smaller-grained, there is more 

 banded felspar, and the mica appears more commonly to have 

 passed over into chlorite, giving this portion a singular resemblance 

 to the rock of Bryn-y-garn, St. David's *. At the western summit 

 or Garn, we appear to be near the edge, as bands of broken material 

 render the aspect somewhat fissile, but the rock maintains its 

 character, though somewhat broken. 



On the western base it has put on a very different aspect : it is 

 coarse again, the quartz is less abundant, and the chlorite derived 

 from mica is in large conspicuous patches. This is on the border of 

 the mass. Further south, at Llangwnadl, we find more border-types 

 in which the quartz is first arranged in strings, so as to give a 

 gneissic appearance, and the chlorite seems to have originated in 

 epidote ; and next, the rock is so full of hornblende and chlorite that 

 it becomes a hornblende-schist, but still retains its quartz, differing 

 therein from most of those which have been called diorites. 



Similar phenomena appear on the shoulder called Bwlch-j^-clawdd, 

 overlooking Meillouydd farm, in the mass on the south. This is 

 never quite so clean, and always contains a certain proportion of 

 black mica, so that it may fairly be called a granite. There is 

 no sign of foliation in the main mass, but on the slopes it becomes 

 beautifully banded. JSTear at hand where broken bands are found 

 in it, the quartz is again reduced to a minimum, and epidote and 

 hornblende are added to the mica. As these phenomena of folia- 

 tion, and the introduction of new, more basic minerals, are in 

 both cases confined to the edge of the mass, dying off in 20 yards 

 or so from the adjacent ashes, I regard them as due to the contact 

 by which some portion of the surrounding material was introduced 

 into the substance of the intruder, and the arrangement common to 

 the edges of large volcanic dykes was brought about. All these 

 rocks are granitic in structure, and the only place where I have seen 

 anjiihing like a felsite in connexion with them is on the eastern 

 edge near Meylltern t* 



Besides these masses, which are marked on the Survey map, there 

 is a patch of a rather different kind of granite, inasmuch as its mica 

 is white, along the road from Llwydiartan southwards, and forming 

 the eastern margin of Mynydd Tstum. A remarkable feature in 

 this is that though so near the other granites, which show scarcely 



* As this is still true, thougk the rock is now proved not to be Pre-Oambrian, 

 it shows how cautious we must be in judging age by rock-character in such a 

 ease as this. 



t See Hicks, Quart. Journ, Geol. Soc. vol. xxxv. 



