482 . EEV. J. F. BLAEE ON THE 



nearly pure fibrous serpentine with isolated spots of black dust. 

 The water necessary for its production must have carried away with 

 it the superfluous magnesia; and this we find impregnating the 

 schists all round, in most places partially, but at the southern end 

 so thoroughly as to produce soapstone. The limestones have every 

 appearance of belonging to the original series and of not being 

 connected with subsequent infiltrations from Carboniferous Lime- 

 stone, which we have no reason to believe ever lay directly above 

 them. We have seen limestone in the older part of the series at 

 Porth Delise ; other occurrences will be noticed in the central dis- 

 trict, and in the higher portions sporadic limestones of similar 

 mode of occurrence are common, in association with volcanic pro- 

 ducts, so we may expect them at this spot. The action of magnesian 

 waters on them was a subsequent process, and they yielded to them 

 some of their own materials ; for on the one hand we find the lime- 

 stones impregnated, especially in the cracks, with serpentine, and on 

 the other the serpentine in their neighbourhood is highly charged 

 with dolomite (?). 



The Gkanite neae Llyn Trefwll. — This is the spot on which 

 Dr. Callaway chiefly relies for proof that there are two Pre-Cambrian 

 series in Anglesey, the one unconformable to the other. He states 

 that here, " within a quarter of an acre," there are " outcrops of 

 three rock-systems — Gneissic, Pebidian, and Palaeozoic, with proof 

 of succession in the order here given." My interpretation of the 

 district is entirely dififerent, and much more nearly coincides with 

 that of the Survey ; though I cannot quite agree with the words of 

 Sir A. Ramsay, who simply says " there is a small patch of granite, 

 around which the rocks are much altered, being interlaced by 

 numerous granite veins." The spot is a very interesting one, and, 

 prima facie, appears to couDtenance Dr. Callaway's description. But 

 the notion of three systems occurring in this way within a quarter 

 of an acre is so incongruous that one is bound to inquire more 

 closely, and then it is soon seen that appearances are delusive. T 

 present a plan of the place (fig. 5), which, if not quite correct, is 

 certainly more so than Dr. Callaway's. The rocks stand out of a 

 grassy surface in long irregular hummocks. On the west side are 

 continuous exposures of the ordinary, not very highly altered, 

 ehloritic schists. The next boss on the east is a tolerably high one, 

 composed of diabase. This is marked greenstone on the Survey 

 Map. It is somewhat foliated here and there, but its character is 

 perfectly distinct under the microscope. To the east of this, and 

 separated from it, is a narrow ridge, of which the central part is 

 granite, and the western and part of the eastern side, at least, 

 is diabase. Dr. Callaway says there are greenish grits on the 

 eastern side ; there may be, but I did not observe them. The 

 granite is of the same type as that seen at Pen-bryn-yr-Eglwys, 

 but is broken up in places, and tinged with infusions of chlorite. 

 A larger ridge appears on the S.E. side; on the western side of 

 which there is a continuation of the diabase, and on the eastern side 



