Vol. SS-] 



GEOLOGY OP NOKTHEEN ANGLESEY. 



671 



regards the confusion and irregularity as an appearance caused by 

 * the underground branchings of the quartz-knob.' ^ Considering 

 that all the other rocks in the Bay are shattered and displaced, we 

 may safely assign this effect to disturbance. Its diff'erence from 

 the ordinary crush-conglomerates consists mainly in this, that the 

 latter ard derived from soft rocks with subordinate hard layers, 

 while here the predominating rocks are hard, with only subordinate 

 soft layers. 



Pig, 12. — Quarry on the east side of Craig Wen, looJcing north : the 

 section coincides ajoproocimateh/ with the strike of the beds. 



W ,^ sli/jht E 



^._.v..d-fe 



slight 

 bend 



concealed ^-^^ij* *.\ 

 l}y broJocTi stone 



[The line-shading denotes the shales, crushed into the quartzite, which is 

 indicated by small crosses.] 



We see, then, that the ' quartz-knobs ' of Forth Llanlliana and 

 ■Oraig Wen are long ridges of a pure quartzite, which is built up — 

 in part, at any rate — of clastic elements, shows bedding, contains 

 bands of shale, and exhibits evidence of crushing and movement by 

 which it has (to some extent) been broken up. Further movement 

 might divide such ridges into a series of detached lenticles or ' knobs.' 

 This is what I consider has taken place between Forth Wen and 

 Bull Bay, and in other parts of this northern area where quartz- 

 rock and limestone form strips or occur as lenticular, elliptical, or 

 rounded masses along a zone which has a rough agreement with 

 the general strike. A series of small • knobs ' of limestone and 

 quartzite is exposed south of Bull Bay village. The farmer who 

 worked the land told me that one of the quartzite-knobs had been 

 entirely removed for road-metal, that it ' did not run deep,' but 

 was surrounded everywhere by the country-rock. This statement 

 strengthens the opinion that these masses are mere lenticles along 

 lines of movement, and is confirmatory of the explanation that 

 Mr. Greenly ^ has given of the ' quartz-knobs ' of Fen y pare, near 

 Beaumaris. 



Microscopic sections from the crush-zones reveal all the successive 



1 Quart. Journ. Geol. Soc. vol. xliv (1888) p. 521. 



2 ' Quartzite-Lenticles in Schists of S.E. Anglesey,' Geol. Mag. 1896, p. 551. 



