ON THE OLDER ROCKS OF ANGLESEY. 407 



In the extreme north the rocks are of the same character aa the last 

 described group, but not glaucophane bearing ; but their regularity is 

 much interfered with, and the cracks and intervals are filled with fresh 

 quartz or felspar. This is well seen in the rock at Ty Lon (185) and on 

 the summit of Mynydd Llwyddiart (185), and in these perhaps we see 

 the maximum change of the series. 



4. The Gabhros and Serpentines. — These rocks having been already 

 systematically studied and described by Professor Bonney, there is the 

 less necessity for dealing with them at length. The existence of gabbro 

 as well as serpentine in the area has been proved, and the derivation of 

 the latter from olivine-bearing rocks has been demonstrated. Neverthe- 

 less, the present examination has brought out some additional points of 

 interest. The gabbro at Ty Newydd, near Four Mile Bridge (45), is a 

 well-marked rock. The numerous diallage crystals now stand out 

 porphyritically from the ground mass ; they are fairly well preserved, and 

 have secondary enlargements which are not optically continuous with 

 the original crystal. Two minerals at least contributed to the grouud- 

 inass. One, which was probably the felspar, is now represented by 

 opaque dust, the crystalline aspect of which with the paraboloid sug- 

 gests its previous conversion into epidote. The other mineral has pro- 

 duced transparent patches, which show a fine fibrous or closely cleaved 

 structure, parallel to which it extinguishes. It may therefore have been 

 an enstatite. 



Starting with this normal rook we may now by several steps trace 

 its passage under the action of pressure and shearing into a schist which 

 may easily be mistaken for one of the ordinary schists of the island ; 

 thus confirming the suspicion of Professor Bonney with regard to the 

 schistose rock of Tyddyn Gob, that it might be ' an altered gabbro with 

 pressure foliation.' 



In the rock at Dinas Fawr (46) the diallage looks still more por- 

 phyritic from the groundmass having passed over entirely into a 

 microspectral mass, in which all individualisation of the original fels- 

 pathic crystals is entirely lost, and the whole has become saussuritic ; 

 where it has been pulled asunder the interstices are filled up by growth 

 from the surrounding diallage crystals. The indestructibility of the 

 diallage in the midst of these changes is noteworthy, and assists greatly 

 in the chain of evidence which leads us at last to the schists. The 

 next link is at the same spot, and forming one large knob with the last 

 (47). This example is almost entirely composed of fine crystalline 

 strings, tailing ofi", or interosculating with each other, and having the 

 pulled-out look characteristic of mylonites ; but amongst these are some 

 distorted fragments of diallage. As diallage has nowhere else been 

 found than in the gabbros, its occurrence may be taken as evidence that 

 the rock has been derived from its neighbour by mechanical deformation. 

 The proof of this depends on the indestructibility of the diallage. But 

 the diallage is not entirely indestructible. In the former rock the 

 slight alteration of the diallage produces crystalline fibres, and in the 

 latter the abundance of these is correlated to the few relics of the original 

 mineral. These fibres appear to be of the same nature as those which 

 produce the ' soapstone,' so that we may call them talcose. The origin 

 of these talcose schists at this spot is certainly from diallage-bearing 

 rocks. We must therefore be prepared for further chemical changes. 

 In the next rock of the series, at Yr Hendty (48), the diallage is entirely 



