MM. Cole & Holland— Structure of Mobell Faivr. 451 



about 250 feet above the Afon Mawddach, or 1200 feet above the 

 sea. The beds dip from 25° to 30° to the north-east, and. if con- 

 tinued across the valley, would rise easily towards the crest of 

 Ehobell Fawr. Below the grit is a series of imperfect slates, re- 

 garded by the Survey as Tremadoc ; while beneath these, on the 

 opposing slope, undoubted Lingula Flags occur. Above the main 

 grit-band come about seven feet of slates ; then a thin upper seam 

 of grit. Both these beds, as on Moel Cors-y-garnedd, contain black 

 argillaceous pebbles as well as quartz. Higher up follow more 

 slates, and then finely-bedded ashes like those associated with the 

 grit on Ehobell Fawr. Lastly we have the well-marked Arenig 

 volcanic series, with its grey massive " felstones " or eurites, forming 

 the dip-slope of Allt Lwyd and the summit-cliff of Ddualt, the 

 latter lying immediately to the south. 



We have found a pebble of pisolitic iron-ore high upon Allt 

 Lwyd, and the bed occurs in situ below the eurite on the hill east 

 of Llanfachreth. 



The base of the ashes and agglomerates of Ehobell Fawr ad- 

 mittedly rests upon Lingula Flags. The slaty beds of this series, 

 with the peculiar black streak mentioned by Belt as characteristic of 

 his "Dolgelly Group," may be traced all round the area, and notably 

 upon the north near Ehobell-y-Big. In the beautiful little gorge 

 of the Afon Geirw, the igneous rocks are distinctly intrusive, and 

 the Lingula series is considerably contorted and crumpled ; but in 

 the moor east of Ehobell-y-Big the hornblendic ashes that con- 

 stitute the remarkable basement-layers of the volcano can be seen 

 in direct contact with slates containing Orthis lenticularis and 

 numerous fragmentary remains of small Trilobites. Careful search 

 in this district should reveal many of the fossil forms already known 

 from Moel Hafod-Owen. 



This junction with the volcanic series is well exposed in the little 

 stream-cut, about 1300 feet above the sea, descending to the Maw- 

 ddach at Dol-y-cynafon. The great mass of hornblendic ash and tuff 

 rises above, as is clearly shown at Graig Fach some two miles 

 further to the south. As Professor Bamsay has pointed out, there 

 is no evidence to connect the volcanic series with the Lingula Flags. 

 It occurs upon the highest members of that series, without the 

 intervention of Tremadoc beds ; but at the same time it differs in 

 lithological characters from the adjacent volcanic products that 

 undoubtedly overlie the Arenig grit. 



The apparent thinness of the Tremadoc Slates in this area has 

 caused some trouble to stratigraphers ; but, supposing a fault to 

 occur along the foot of Allt Lwyd and of Ddualt, this only serves 

 to increase the difference of level and of relations between the series 

 of Ehobell Fawr and that of the Arenig " felstones." The position 

 of the grit-beds on Ehobell Fawr seems to greatly strengthen our 

 contention, viz. that this mass represents a volcanic outbreak 

 occurring between the deposition of the latest Lingula Flags and 

 the Arenig grit, and consequently of Tremadoc age. 



The earliest outbursts were doubtless accompanied by considerable 



