378 KEPORT — 1888. 



no longer axis, or the different crystals are arranged in different directions, 

 there is no orientation. The latter of these two alternatives is usually 

 confined to igueous rocks ; and hence we find that such sedimentary 

 rocks as are not orientated have isodiametric elements, and thus form 

 a mosaic. The term mosaic felspar has been used when the elements 

 belono- to that family, but there are other minerals of the same habit, 

 under similar circumstances, so that we may extend the term and speak 

 of mosaic rocks. 



The mosaic rocks arise principally by the alteration of those of fine 

 original grain, and form a very characteristic group. We may take as 

 examples of these the rock at Llanfaethlu (27), Llaurhyddlad (30), and 

 the hill over Ogo Lowry (32) (see fig. 4). When the most unbroken 

 parts of these are examined with the paraboloid they are seen to consist 

 of a transparent mass, in which there float very minute specks of reflect- 

 ing matter. These specks are comparable in size with the finest cavities 

 in quartz, or those which give a slight opacity to a translucent felspar. 

 Where the grain of the rock is smallest, the specks appear uniformly 

 distributed ; but in the slightly coarser parts they are aggregated into 

 minute patches separated by clearer lines. These aggregates are seen 

 between crossed Nicols to represent the individual crystals, and when 

 they appear uniformly distributed the crystals are too small to be indi- 

 vidualised. Amongst these may float more or fewer crystals of chlorite, 

 so minute as to be visible only under a high power, and quite in- 

 distinguishable between Nicols. This kind of rock appears specially 

 liable to be cracked and torn — the cracks being of more than one age, 

 and sometimes filled with larger crystals of quartz, and sometimes only 

 with dust — and these again are twisted and bent. Such is the minute 

 structure of the rock, which has been called a 'marbled slate.' It is of 

 wide occurrence in Anglesey. 



Yery nearly allied to this type of altered rock are those which con- 

 tain angular fragments in a fine groundmass. This groundmass has 

 become a fine crystalline mosaic, but here and there is seen a peculiar 

 kind of discontinuity which may be called a tension area. Such areas 

 are more or less in the form of three-rayed stars of very irregular 

 form, and they are occupied by elements of larger size than the rest. 

 Now, wherever an ordinary crack traverses a rock, the elements with 

 which it is filled up are of medium size ; and therefore larger than the 

 finest. We may suppose, therefore, that these irregular areas, which die 

 out on all sides, represent spots where, as in cracks, the pressure has been 

 relieved. Kocks showing this structure have been examined from near the 

 river Alaw (22), at Caer-deon, near Llanddeussant (24), near Cemmaes 

 (216), at Llechog Ucha (221), Pengorphwyfsa (222), Llaneilian (223), 

 and Point ^lianus (224). In the last case there is an apparent orienta- 

 tion, but it is found not to affect the substance of the rock, but to be due 

 to a number of later parallel cracks which have been filled with sericite. 

 This is the ' foliation oblique to the bedding ' of Sir A. Ramsay. 



From these we are led on to a type of rock which is very difiicult 

 to explain. It consists entirely of unorientated elements of medium 

 size, large enough to show sutural junctions. These are not all of one 

 kind, but apparently consist of felspar and quartz in equal proportions. 

 The best of these is the remarkable rock associated with the granite 

 to the west of Gwalchmai (86), and usually called a hiiUeflinta ; except, 

 for its apparent bedding on the large scale it might almost be a felsite. 



