388 REPORT— 1888. 



lines are admirably shown in many of the Anglesey rocks. In some 

 they occur as simple lines of dust, which may give a pseudo-laminated, 

 or slaty aspect to the rock, as at Forth Dryw (12). A more remarkable 

 example is in the north-east of Malldraeth Marsh (190). The rock has 

 all the aspect of a purple slate, and, indeed, both it and the rock at Forth 

 Dryw may never have been anything else. But at present the mylonitic 

 lines obscure all other features, the whole rock being mylonised, and the 

 intervening substance, though very minute, is microspectrally polarised. 

 In the rock at Fen-y-Farc, Beaumaris (193), they have to pass round the 

 numerous small fragments of quartz, which they do in a characteristic 

 way, and they are largely converted into frayed-out sericite (see fig. 17). In 

 the rock at Tyddyn, north of Beaumaris (199), they are so abundant that 

 with the frayed-out sericite they occupy broad bands, leaving only 

 small intervening portions, in which microspectral polarisation may be 

 observed. In most of these cases special causes may be suggested that 

 should cause shearing in these localities. In other cases the lines are now 

 entirely crystalline, but can still be recognised by their other character- 

 istics, and by the fraying out of the ends of the sericite. In this state 

 they are well seen in the slaty-looking rock at Forth- da- farch, Holyhead 

 (61), at Clawd-y-parc, Llandegfan (196), at Coedmawr, Llanfaes (200), 

 and at Llechog Ucha, Amlwch (221). In some cases the lines are so 

 abundant and regular that they may probably coincide with the original 

 lines of lamination, which have been drawn out in their own direction, 

 as beyond Forth Dryw (13), and at Font Scyphydd, Llanddeus.sant (25). 



KOCKS OF SPECIAL ORIGIN. 



Amongst the ordinary sedimentary rocks of Anglesey are found cer- 

 tain special types which differ from the rest both in their mineralogical 

 composition and in their mode of occurrence. These are found in isolated 

 masses, often of peculiar form, and are quite characteristic of the series 

 to which they belong. Their origin must be partly determined by theiv 

 stratigraphy, but their microscopic structure may throw some light upon 

 the question. The two principal types are limestone and quartz rocks. 



Limestones. — These are of several types, some of which have been 

 already described by Frofessor Bonney. 



One form of limestone, which occurs, like all the rest, in isolated or 

 lenticular patches, is found amongst the crystalline rocks of the Central 

 district, and partakes of their character. Since the rocks amongst which 

 they are found have been recrystallised, these have doubtless been also, 

 especially as they are distinguished from the rest by the large size of their 

 elements, which vary from '02 to -04 inch in diameter. The crystals are 

 characteristically twinned, and occupy the mass of the rock, with only a 

 few grains of quartz, mica, or black dust in the interstices. The one at 

 Trecastle (90), discovered by Dr. Callaway, is orientated with the rest 

 of the schists, but that at Bodwrog (91), inserted on the Survey maps, is 

 disturbed and irregular. These were, doubtless, like the other varieties 

 before their recrystallisation, and may be considered to have had a 

 similar origin. There are limestones also in the older or more stratified! 

 portion of the series in the Western district. One at Forth Delise (15) 

 occurs in an irregular tongue in a disturbed area, and is doubtless out of 

 its original place. Its later alteration is well marked by the abundance 

 of large crystals of mica associated with the calcite ; another at Cruglas, 



