668 Geological Society : — 



apparently Upper Llandeilo age. The general dip of all the 

 rocks is northerly and north-easterly. 



The Garn Phyllites are usually green altered shales and fine 

 gritty rocks, and are intensely contorted near their southern 

 boundary. Even where not contorted they show under the micro- 

 scope evidence of powerful earth-movement. They are considered 

 by the author to be part of the ' Green Series ' of Northern 

 Anglesey. They are cut off to the west and south by a curved 

 fault, probably a thrust, which brings them against Llandeilo slates 

 and breccias. 



The Garn Conglomerate, Grit, and Breccia, a formation 

 perhaps 400 feet thick, rests upon the Garn Phyllites and contains 

 fragments derived from them, as well as pebbles of quartz, grit, 

 gneissose and granitic rocks, etc. It passes up gradually into 

 black slates, from which a few Upper Llandeilo fossils have been 

 collected. In the black slates an oolitic ironstone or ferruginous 

 mudstone has been found, which may perhaps be on the same horizon 

 as the similar rock recorded by the author in Northern Anglesey. 



On the eastern side of Mynydd-y-Garn is another group of rocks, 

 the Llanfair-y'nghornwy Beds, which the author correlates 

 with the basal part of his Llanbadrig Series. They consist of 

 phyllites resembling those below the Garn Conglomerate, but they 

 contain also beds and masses of quartzite, grit, and limestone. 

 They are much broken, and partly in the condition of crush-con- 

 glomerates. They have been thrust over the Llandeilo black slates, 

 and the thrust-plane has been traced to the coast at Porth yr Ebol. 

 This thrust is continuous with that which forms the southern 

 boundary of the ' Green Series ' of Northern Anglesey. 



The district around Mynydd-y-Garn has been affected since 

 Llandeilo times by two powerful earth-movements, acting one from 

 the north, the other from the north-east. The first-mentioned 

 prevailed in the area west and north-west of the hill, where the 

 pre-Llandeilo rocks are frequently shattered to crush-conglomerates. 

 Around Mynydd-y-Garn itself and east of it the principal direction 

 of movement has been from the north-east ; south of the hill the 

 structure is perhaps the result of the interference of these two 

 movements. 



3. 'On some Altered Tufaceous Rhyolitic Rocks from Dufton 

 Pike (Westmorland).' By Frank Rutley, Esq., F.G.S. With 

 Analyses by Philip Holland, Esq., E.I.C., E.C.S. 



The speoimens described were collected by the late Prof. Green 

 and Mr. G. J. Goodchild from the Borrowdale volcanic series which 

 constitutes the central mass of Dufton Pike, and the chief interest 

 attaching to them is their alteration, probably as the result of 

 solfataric action. One of the rocks, which has the composition of 

 a soda-rhyolite, contains felspar, augite, magnetite, and possibly 

 spinel or garnet, scapolite, and ilmenite. The porphyritic crystals of 

 felspar are much corroded, and are sometimes mere spongy masses in 

 which mica and opal-silica have been developed, together with small 

 quantities of carbonates, In a second example, felspar-fragments 



