Vol. 52.] LINGT7LA-FLAGS AND IGNEOUS ROCKS OF DOLGELLY. 513 



Lastly, we have some reason to believe that a fault runs nearly 

 from north to south just outside our map on the east ; but our 

 evidence is not complete, and in any case the fault does not affect 

 the area actually described. 



In the north-western corner of the map the dip is southerly.. 

 The Middle Lingula-beds are well exposed and are overlain, just 

 along the northern margin of the diabase, by a band of darker 

 slates belonging to Belt's ' Dolgelly Group/ 



The Derwas fault clearly has its downthrow on the south-eastern- 

 side, and brings the black slates and lava-band of the Upper 

 Lingula-beds into line with the Middle Lingula-beds on its north- 

 western side. But it does not affect the inclination of the beds,, 

 for the lie of the lava in the Bryn-y-gwin woods shows that the 

 general dip is still southerly. This is maintained even on the other 

 side of the great diabase mass, for near Graig we find slates and 

 ashy beds, the latter showing a dip of about 32° due S. 



East of the Dolgelly fault, and north of Want Ceunant, most of 

 the surface is occupied by diabase ; but the ashes and other stratified 

 deposits which form the band stretching across towards the Afon 

 Aran still keep the southerly dip. 



South of the Kant Ceunant fault, however, there seems to be a 

 marked change in the general strike. On the northern slopes of 

 Mynydd Gader the band of lava in the Upper Lingula-beds runs 

 about W.N.W.-E.S.E., indicating (since the surface slopes down 

 from south to north) that the beds dip towards the south-west. 

 In the Middle Lingula-beds, however, north-east of this lava-band, 

 there is considerable variation, and south-east of Brynmawr the 

 dip is due west. 



IY. The Stratified Rocks. 



(a) The Slopes of Mynydd Gader. 



Nant Ceunant. — The fullest and clearest section of the sedi- 

 mentary strata in this district is to be seen in Nant Ceunant, a 

 tributary of the Afon Aran. 



About 250 yards N.E. of Tan-y-fedw the diabase, which forms 

 most of the bed of the stream up to this point, gives place to bluish 

 slates, interstratified with hard gritty bands. These continue up- 

 the stream for some distance, and contain Lingulella JDavisii in 

 abundance. They belong to Belt's ' Ffestiniog Group/ 



Gradually the slates become darker and darker in colour, and 

 cease to yield Lingulella ; but a solitary fragment of a trilobite was 

 discovered. A narrow band of volcanic ash or lava (not shown in 

 the map) occurs ; and then follow more dark slates extending to a 

 distance of | mile south of Tan-y-fedw, where they are succeeded 

 by the thicker bed of andesitic lava shown in the map. 



This lava is followed by a series of very black fissile slates with a 

 black streak, which continue without much change up to the crags 

 of Mynydd Gader. But at one point we came across a small band 

 of ashy-looking rock, which appears to strike in quite a different 



