278 TROF. T. G. BONNEY AND MISS C. A. RAISIN ON [May 189O, 



penline ^) some dull greyish-green rocks appear to include a compact 

 diabase and a streaky, probably crushed, gabbrj (see p. 293). 



South of the streamlet and went of Llyn Pcnrbyn, several large 

 bosses occur, some composed of serpentine, some including other 

 rocks (chlorite-schist, gabbro, etc.) ; ^ but from this place all the 

 ground southward to the railway, and westward towards Trciiesg, 

 is mainly occupied by dull pale-green ' schists,' sometimes with 

 crumpled laminas, sometimes with lustrous sheen-surfaces, occa- 

 sionally quarried. 



Gabbro forms abrupt masses (where Yr-ynys is built), while east 

 of Caegias some serpentine and ' ophicalcite ' occur, with a littl*^ 

 chlorite-schist. The gabbro and the green schists can be traced 

 across the railway, and the serpentine may also extend to the field 

 beyond, where it is quarried. Craglet-^, at first of schist, afterwards 

 of gabbro, project above the low meadows, and lead to the 1 irge 

 3j\ass of gabbro which rises abrupt!}' with a plateau-like summit 

 towards Felin-carnau. 



(3) Mainland of Anglesey — south of Llanfair -yn-neubwll 



* Inlet. 



A small patch of serpentine (much crushed) forms a low cliff and 

 the rock on the beach south-west of Felin-wen.^ North of it a 

 modified diallage-rock (containing some enstatite) occurs, which is 

 probably a dyke ; while a brecciated dolomitic limestone is found a 

 little farther southward along the shore, and at the western end of 

 the causeway near Penryn-hwled. A small boss of serpentine and 

 limestone, or ' ophicalcite,' is quarried in a field east of the road 

 south of Plas.* 



(4) Holyhead Island — Khoscolyn Parish. 



(«) General Description, and Boundary -line. 



On the west of the ' strait' is the larger district of serpentine and 

 associated rocks which form a somewhat tongue-shaped area, about 

 2 miles from east to west and g mile from north to south. The 

 boundary is irregular, and the rocks in places, especially on the 

 bouth, rise abruptly from marshy land. To the north, the schists 

 are well exposed in large bosses, but among them, one small piece 

 of serpentine is quarried immediately north of the Holyhead road 

 (noith of Plas-rhyd-bont). The serpentine and other rocks often 

 show, towards the boundary, a brecciated and even a smashed cha- 

 racter, and the schistose planes (like the laminae of the schists) 

 generally dip northward at a moderate angle. Thus the boundary 

 may correspond, partially at least, with a thrust-plane. 



^ This rock, like many masses, e^pef^ially of serpentine or of chlorite-schist, 

 aifords good examples of surfaces polished by sheep rubbing against them, 



'^ One juncrion is discussed on p. 295. 



^ Described as near Ty-ucha (on the l-in.-h map) in Quart. Journ. Geol. 

 Soc. vol. xxxvii (1881) p. 41. 



^ Prob .bly that ' near Gwrthya,' ibid. p. 42. 



