Vol. 55.] GEOLOGY OF NORTHERN ANGLESEY. 655 



the area north of Craig Wen, where folds in the conglomerates and 

 grits are often concealed by cleavage-planes, I noticed a divergence 

 of some 15° between cleavage-strike and bedding-strike. Weathering 

 sometimes brings out other systems of close-jointing, which may be 

 earlier cleavages more or less obliterated by the pressure which 

 produced the principal cleavage. For instance, the limestone and 

 jaspery masses in the ' crush-conglomerates ' at Penrhyn, near 

 Cemaes, have such a set of vertical planes at right angles to the 

 already-noticed cleavage-strike. A third set is sometimes noticeable 

 agreeing in strike with the principal cleavage, but dipping north- 

 ward at a much gentler angle. The facts suggest a series of move- 

 ments in Anglesey very similar to those that produced the three 

 cleavage-systems noticed by Mr. Lamplugh in the Isle of Man ; but 

 the point requires further attention. 



{d) Crushing. 



In the ordinary parallel cleavage described under the last head 

 there is little or no sliding movement, the particles of the rock 

 retaining as a whole the same position relatively to their neighbours 

 before and after the compression. A further stage in the effects of 

 compression is the differential movement of the molecules of the 

 rock, causing them to slide past each other, and resulting as a rule 

 in a phacoidal structure and a wavy cleavage. This breaking- down 

 of the original structure by shearing movements is usually accom- 

 panied by a development of a micaceous mineral, such as sericite, 

 that renders the divisional planes glossy. Hard and resistant 

 fragments included in the rock tend to be puUed out into phacoids, 

 or to be rolled out into strings, or to break up into minor fragments. 

 All stages up to complete mjionization may be observed. 



The breaking-up of rocks into phacoids is best known on the 

 minute, but occurs also on the macroscopic scale. A good example 

 is to be seen south of the graptolitic shales of Perth Padrig, in a 

 yellowish-brown sandy rock. Again, a mass of limestone on the 

 coast 250 yards north of Ogof Gynfor is similarly divided by 

 curved joints into lenticles. These are not by any means solitary 

 instances. 



On the smaller scale we have an ' augen '-structure developed in 

 fine grits, as, for example, in Perth Padrig (underneath a broken 

 overfold, N.A. 9) and in the older beds of the Ogof Gynfor uncon- 

 formity (IS". A. 46 & 47). Similar structures are common north of 

 Craig Wen, and interbedded with the Craig Wen quartzite are shales 

 which in part are practically mylonites (N.A. 43). 



Compression and shearing have had a marked effect on some of the 

 conglomerates of the northern coast, and the results may be studied 

 on both sides of Perth Wen Bay and in some localities inland. The 

 pebbles have been sometimes flattened, sometimes pulled out into 

 long phacoids or even strings of quartz or quartzite, at other times 

 chopped into pieces by parallel planes of movement and thoroughly 

 brecciated. 



