Vol. 69.] PETROGRAPHY OF BARDSEY ISLAND. 531 



Maen Dii [9306], contains a good deal of felspar in very minute 

 grains, and proves to be an adinole. It is readily fusible before 

 the blowpipe, and represents a shale altered by contact with an 

 intrusive diabase. 



EXPLANATION OF PLATES XLIX & L. 



Plate XLIX. 



Eig. 1. Interbedded bands passing into ' crush-conglomerate,' as seen in the 

 coast-section near St. Mary's Well, south-west of Aberdaron. Tbe 

 photograph also shows the deformation of a massive bed of quartzite 

 into two wedges. (See p. 524.) 

 2. ' Quartz-knob' on the western slope of Mynydd Enlli. (See p. 522.) 



Flate L. 



Geological map of Bardsey Island, on the scale of 12 inches to the 

 mile, or 1 : 5280. 



Discussion. 



Mr. G. Barrow said that he regarded the paper as of extreme 

 importance, as it dealt with one of the torn-out and piled-up masses 

 of rock that accompanied the movement of great crystalline 

 portions of the earth's crust. For the meaning of it one had to go 

 to Anglesey, where there was one of these great crystalline areas, 

 within which newer rocks of varying ages had been dropped down 

 by faults. Of these, the variolites or pillow-lavas, mentioned by 

 the Author, had been described by Mr. Greenly in a paper read 

 before the Geological Society. On the north-west side of the Menai 

 Straits denudation had laid bare the base of the heaped-up mass, 

 the last remnant of which was seen there in contact with the outer 

 or uncrystalline margin of the Archaean area. The great crystal- 

 line mass had snapped off at this margin, and a thrust-plane had 

 developed. As the mass slowly glided along this it bent, buckled, 

 and finally broke up into lenticles the rocks that lay in its path, 

 heaping them up, and forming the confused medley of rocks 

 described by the Author. The phenomena were similar to those 

 met with along the southern Highland Border (seen in the North 

 Esk), and along portions of the Moine Thrust. All were, connected 

 with, and marked the position of, the outer margin of crystallization 

 ■of one of the older Archaean complexes. 



Mr. W. G. Fearnsides wished to associate himself with the con- 

 gratulations to the Author expressed by the previous speaker. The 

 detailed record of the structures exhibited by the old rocks in 

 Bardsey would supply a want, long felt by workers who were 

 trying to puzzle out structures in the less well-exposed districts of 

 the mainland of Wales. 



To the speaker, the map exhibited suggested very strongly that 

 the Bardsey rocks had been subjected to at least two sets of earth- 

 movement. To the earlier set might be referred the movements 

 which crushed up the sedimentary series into shallow isoclinal 



