42 



PROF. T. G. BONNET ON THE SERPENTINE 



schist and serpentine are so much affected by subsequent changes 

 that it is difficult to be sure. 



On the north-eastern edge of this massif of serpentine (Survey 

 map), near Cruglas Parm, five shallow pits have been opened in a 

 low ridge east of the road, two being close together by the road 

 and outside the enclosure. In one the rock is a normal serpentine, 

 as it is on the right-hand side of the other ; but on the left we find a 

 tougher variety (see below, p. 46). I failed, however, to find any 

 line of demarcation between the two varieties. The next two pits 

 are in normal serpentine, veined in places with calcite. The last is 

 a kind of ophicalcite, hard and tough. On my first visit I found a 

 loose block, near one of the western pits, of a dark green serpentine 

 full of lustrous crystals, up to 0-3 inch diameter, of a mineral of 

 nearly the same colour. I could not then discover any of this rock 

 in situ, and on a second visit, after a most careful search, aided by 

 some of my students, was equally unsuccessful, though in one or 

 two places the presence of a few minute crystals in the normal 

 serpentine' indicated a slight approach to it*. On the western side 

 of the road a pit has been opened in a large mass of gabbro, some 

 of which has a rather serpentinous aspect. 



About a quarter of a mile further down the road we passed over a 

 reddish serpentine, rather brecciated and veined with calcite. We 

 again touched a little serpentine (dull green) near Gwrthya. All 

 the south-western end, adjacent to the shore, of the large mass near 

 Melin Carnau, coloured as serpentine on the map, is gabbro. 

 Thence we worked along the shore northward till opposite Ynyslas. 

 There is no serpentine here, the gabbro being succeeded by schists, 

 some of which are, indeed, massive and serpentinous in aspect; but 

 the difference is evident, even in the field, on careful examination. I 

 failed to find any serpentine at the shore end of the slope opposite 

 Ynyslas ; the rock there is the green serpentinous-looking schist ; nor 

 did I see any in the isolated crags inland in the immediate neigh- 

 bourhood. The railway crosses the mass which is represented on 

 the map as running southward from Llyn Treflas; I walked along 

 this, and found nothing in the cuttings but the green schist succeeded 

 by the ordinary schists of the district. 



A much larger mass of serpentine than any of the above is indi- 

 cated on the map near Ehoscolyn, at the south end of Holyhead 

 island. Specimens from this locality are familiar to me ; but I have 

 only examined the western portion of it in the field. As on the 

 other side of the water, both schist and serpentine crop out here and 

 there in rough knolls and ridges among the fields. The two most 

 interesting exposures are on the western side of the western of two 

 roads leading to Ehoscolyn. The first, near Ceryg Moelion, is a 

 quarry approached by a driftway. Along this we pass a large mass 

 of gabbro, which is succeeded by an outcrop of dark green serpentine ; 

 and beyond this we come to the quarry. It has been opened in a mass 



* See below, p. 45, for a microscopic description. 



