STETJCTUEE OF SOME ANGLESEY EOCKS. 



235 



*' streaky " structure, not unlike some of those which occur in the 

 Borrowdale series, containing numerous microliths of the viridite 

 group, some being certainly chlorite. The aspect of the rock suggests 

 that it has undergone considerable pressure. Many of the imbedded 

 fragments are from about 0"'03 to 0"*06 in longer diameter. Among 

 them quartz, felspar, altered biotite ('?), and a chloritic quartz-schist 

 may be recognized, detrital materials almost certainly derived from 

 the older gneissic and schist rocks of this region of North Wales. 

 Other fragments of a less certain character are present, with grains 

 of decomposed ilinenite or magnetite and of epidote, which perhaps 

 has replaced some other mineral. 



48. (Base of Point JElianus, p. 223.) A rock of a generally similar 

 character ; but, as the fragments are smaller, their nature (except in 

 the case of the quartz) is less easily ascertained ; the whole rock also 

 seems slightly more altered than in the last case. 



50. (Coast N.W. of Amlwch, p. 223.) The materials are more 

 homogeneous than in the other cases ; small greyish clustered 

 granules frequent, and a vast number of microliths of a green 

 mineral (chlorite ?). These are very likely of secondary origin, but 

 I consider the rock a true slate, and not a schist. 



42. (S.E. of Cerrig ddwyffordd, p. 214.) A fine grit with much 

 of the fibrous green (? hornblendic) mineral which I have often ob- 

 served in the Charnwood rocks developed among the finer materials ; 

 the larger fragments (generally rather angular) are chiefly quartz, 

 felspar (orthoclaso and plagioclase), and a minutely micro crystalline 

 or cryptocrystalline rock, much resembling bits of an acid lava. 

 Prom certain minute peculiarities, I have strong suspicions that all 

 three of these constituents have been derived from rocks of volcanic 

 origin. 



38. (Fragment in Llangefni conglomerate, p. 214.) A grit, consist- 

 ing of subangular and rolled grains, mostly quartz, in a fine quartzose 

 or quartzo-f elspathic matrix. It is difiicult in the case of rocks of this 

 character to draw an inference as to their age and amount of meta- 

 morphism ; but I believe that I am right in grouping it with the 

 probably more modem series. 



D. Granitoid Gneiss. 



Of this rock there is one specimen from Pen Bryn yr Eglwys not 

 at all in a favourable condition for microscopic examination. It 

 resembles the group of coarse granitoid gneisses similar to those 

 which I have examined from the neighbourhood of Llanfaelog 

 rather than a true igneous rock. It consists of quartz with many 

 minute enclosures, two felspars much decomposed, and various 

 alteration-products probably replacing an iron-magnesia mica. One 

 or two grains of another mineral are present, obviously much 

 altered, which I cannot identify ; possibly it may have been garnet. 



E. Crystalline Limestones. 



40. (E.N.E. of Tany graig, Pentraeth,p. 213.) A finally granular 

 dear rock consisting almost wholly of calcite or dolomite. Judg- 



