ON THE OLDER BOCKS OF ANGLESEY. 381 



The cleavage in the Holyhead quartzite (1) is not of the ordinary charac- 

 ter, as may be well imagined, seeing that quartzite is not one of the most 

 usually cleaving rocks. It is, however, remarkably orientated with a 

 variety of quincuncial orientation — i.e., instead of isolated crystals being 

 orientated, there are groups of such crystals. These, which are flakes 

 of colourless mica, form a large portion of the rock, and the intervals 

 between them are filled with quartz elements without very clear bound- 

 aries. The orientated mica does not in the least turn aside in the neigh- 

 bourhood of the quartz fragments ; on the contrary, the ends go straight 

 against them, and seem to cut into them, so that their boundaries are 

 jagged. In this rock, therefore, the cleavage is the result of the orienta- 

 tion, and there is no deformation of the particles. We must not, therefore, 

 in this case speak of cleavage-foliation, but rather distinguish this — the 

 foliation — as one cause of cleavage, which may be called foliation-cleavage 

 (see fig. 12). The quartzites of Roscolyn (65), Porth-y-gwalch (64), and 

 other beds in the neighbourhood (66) are also well cleaved, but the 

 cleavage is brought about in a different way. As before noted, the frag- 

 ments are smaller, and they fit closer togethei*, so that there is not room 

 for much mica. What mica is present is quincuncially orientated in the 

 usual way, and would scarcely account for the cleavage. The rocks, how- 

 ever, are affected by a number of cracks, and along these the mica is 

 continuous, and it is doubtless along these also that the rock separates, 

 as is the case at Point ^lianus. This is, therefore, a distinct method in 

 which cleavage may be brought about. Such cleavage might be called 

 fracture-cleavage (see fig. 9). 



In this group there is another remarkable rock, which has been dis- 

 cussed by Sir A. Ramsay — viz., the flaggy bed at the South Stack Light- 

 house (60), which seems to show false-bedding, and foliation along the lines 

 of the false-bedding. In the section of this rock the primary bedding is 

 well shown by lines of brown dust, but when the rock is examined with 

 the paraboloid there is nothing seen between these which should indicate 

 false-bedding. There are, however, a number of parallel oblique lines, 

 scarcely definite enough to be called cracks, in which the mica is con- 

 tinuous ; but the individual elements are not placed in the direction of 

 these lines, not parallel to the bedding, but in a direction intermediate 

 between these two, so that they overlap like a dislocated pack of cards. 

 It is to these oblique lines that the appearance of false-bedding is due ; it 

 is in reality a kind of oblique foliation, whose origin is not very clear. 

 Similar phenomena have been noticed by authors in other districts. 



The foliation of the laminated rocks does not require many words of 

 description. In the Western district it is typically quincuncial, but there 

 is a tendency to pass over into the linear, especially as the elements of the 

 green-mica and chlorite are more closely aggregated in the darker bands 

 of lamination, and the orientation of the individual elements is not very 

 closely defined. In the Eastern district it is more entirely linear, the 

 elements of mica being very irregularly arranged and often of large size. 

 In the rock at Porth-yr-corwgl (6) we have the orientation intensified by 

 the pressure which has taken place perpendicular to its direction. In 

 this rock is seen an interesting phenomenon, illustrative of the origin of 

 authigenetic quartz (see fig. 13). There is a large rounded quartz frag, 

 ment, caught with its longer axis transverse to the lines of lamination, 

 and therefore more or less in the direction of pressure ; one side of this 

 has accordingly broken down, and from the products of fracture there has 



