Vol. 56.] GEOLOGY OF NORTHERN ANGLESEY. 245 



of the quartzite increases, that of the conglomerate decreases, the 

 united width of the two being fairly constant. Pacts such as these 

 point to this quartzite having originated by alteration of the conglo- 

 merate, but I have not felt able to adopt such an explanation, the 

 change from the fine-grained homogeneous quartzite to the adjacent 

 conglomerate being too abrupt. 



The upper part of the conglomeratic beds which occur below 

 the quartzite sometimes has a pale and quartzitic aspect, but its 

 derivation by alteration from a crushed quartzose slaty and pebbly 

 grit is evident. It may have undergone some silicification from the 

 overlying massive uncleaved quartzite, into which it passes rather 

 abruptly. Layers of this character are occasionally seen inter- 

 stratified with the quartzite itself. 



Near Is-allt the quartzite is again associated with a conglomeratic 

 quartzite which I have mapped as basal Ordovician. The quartzites 

 have a conglomeratic appearance in other localities (for example, 

 see PI. XIV, Section 2) ; but in this and some other cases I consider 

 the appearance to be merely the result of brecciation and recementing 

 of the quartzite. 



The mode of occurrence of these rocks in the field, as described 

 above, is difficult to explain satisfactorily. Faulting and move- 

 ment account for some features, as for instance the quartzite- 

 lenticle in the purple conglomerate, the tongue of conglomerate 

 between repetitions of the quartzite, and the cutting-out of most 

 of the conglomerate at Craig Wen ; but the swelling-out of the 

 quartzite at Craig Wen is too great to be assigned wholly to move- 

 ment, though partly an effect of this cause. It is open to us to 

 regard the variations in thickness as the result of erosion of part of 

 the quartzite before the deposition of the conglomerate, a view which 

 has its difficulties; or as an irregularity in the distribution of the 

 original sediment. 



While admitting that these quartzites present features not easy 

 of explanation, I adhere to the opinion expressed in my former 

 paper, that these rocks are altered sandstones which are some- 

 times completely isolated owing to disruption by earth-movements. 

 Ample evidence of their clastic nature is given in Prof. Watts's 

 appendix to that paper ; but it is due to Prof. Bonney to state that 

 the sedimentary origin of a ' quartz-knob ' near Beaumaris was 

 determined by him so long ago as 1883. 1 



(c) Structure of the Northern Complex. 



The mapping of the Northern Complex has brought out in fuller 

 detail the overthrusting from a northerly direction which had 

 been previously demonstrated. The area consists of a series of 

 shattered folds, whose deeper synclines are traceable by means of 



1 Quart. Journ. Greol. Soc. vol. xxxix (1883) p. 471; see also Sollas, Sci. 

 Troc. Koy. Dublin Soc. n. s. vol. vii (1892; p. 169. 



