ON ilANOD AND THE MOELWYNS. 371^ 



felstone above is here at its maximum thickness, ahout 1500 feet. 

 In character it resembles that of Gareg IJdu in its grey colour, 

 cohimnar structure in parts, and slaggy appearance, but it is more 

 por])hyritic. Under the niicrosco])e it is at once seen to differ from 

 the ^loelwyn felstones, and is apparently a true igneous rock, pro- 

 bably contemporaneous. An imperlect spherulitic or granophyric 

 structure is present. 



The strata which formerly covered the felstone on the Manods 

 have been denuded away from the top ; but owing to faults they are 

 found '' troughed in " behind, in the area round the Graig-ddu 

 Quarries, and consist there of agglomeratic rocks like those above 

 Elaenau Fl'estiniog itself. 



V. The Intktjsive Rocks. 



The intrusive rocks belonging to the Ffestiniog district are, with 

 the exception of the Tan-y-grisiau granitite, few and unimportant. 



Between Trawsfynydd and Arenig there are several patches of 

 " greenstone " breaking through the Lingula Flags. As suggested 

 by Sir A. liamsay, these may bear the same relation to the Arenig 

 eruptions that the plutonic masses of Cynicht and Moel Siabod have 

 to those of Bala age. We know of none of this series as occurring 

 within the area under consideration. 



The greenstone near the top of Moelwyn belongs to the Llandeilo 

 series. It is a typical diabase composed of augite, plagioclase, and 

 viridite, with much magnetite ; but though it comes into the 

 Moelwyn area, further consideration of it may be left until the rocks 

 between Ffestiniog and Snowdon can be dealt with together. 



In the Arenig series there are a few dykes cutting through the 

 already metamorphosed strata ; those on the Cwm-Orthin incline, 

 below the Oakley Quarry, and in the mass of Gareg Ddu may be 

 mentioned as examples. There is nothing worthy of notice in 

 the rocks themselves, which are considerably altered andesitic in- 

 trusions. 1'he spotted flags in contact with them are often meta- 

 morphosed to a very compact, mottled brown and green porcellanite 

 with a peculiar lustre. 



The mass of rock known as Moel Tan-y-grisiau, which lies between 

 Ffestiniog and the Moelwyns, would strike almost any observer as 

 ditiering from the surrounding strata. Its rounded form contrasts 

 as strongly with the rugged and broken outline of the Moelwyns 

 as does its covering of purple heather with the cold greys and greens 

 of the surrounding hills. Mapped by the Geological Survey as 

 '' Intrusive Syenite," it has since become better known owing to its 

 inclusion by Dr. Hicks in his list of the pre-Cambrian rocks of the 

 British Isles *. Dr. Hicks at the same time expressed his belief 

 that Mr. Tawney had " most satisfactorily proved that it is mainly 

 of Dimetian age." This view has not been widely adopted, and the 



* Quart. Joiirn. Geol, Soc. vol. xxxv. (J 879) p. 304. In the Description of 

 the Geology of North Wales, written bj the same author for the International 

 Geological Congress of I8b8, it was not, however, included in a similar list. 



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