528 PKOF. W. J. PUGH OX THE GEOLOGY OF THE [vol. lxxix, 



variability in the character of the strata immediately underlying 

 the Mottled Beds is considered to be due to actual lithological 

 variation within the underlying group, evidence of which has 

 already been given when describing the Garnedd-wen Beds. The 

 latter thin out so rapidly from south-west to north-east in the 

 Corris area, that it might be supposed that still farther away to 

 the north-east the Mottled Beds might actually overstep on 

 to the beds with Dicettograptns anceps. An examination of a 

 somewhat larger area than that at present described lends no 

 support to the supposition ; on the contrary, the diminution in 

 thickness is due to a loss of arenaceous material as the Garnedd- 

 wen Beds are traced north-eastwards and eastwards. 



It is considered, therefore, that in the Corris area the Mottled 

 Beds lie conformably on the Ordovician rocks ; but there is an 

 abrupt change in the lithological characters, which indicates some 

 distinct change of phvsical conditions preceding the formation of 

 the Mottled Beds. 



These beds are well exposed in a number of places : as, for 

 example, in the old quarries at Taren y Gesail, where practically 

 the whole thickness of the zone is exposed, on the nose of the 

 Corris Anticline near Cwm Cadian.in the two small streams which 

 drain into the River Dulas south of Corris, near Trem-afon, 

 (north of Garnedd-wen Station), in the River Dulas (east of 

 Aberllefenni), etc. 



The actual junction of the Garnedd-wen Beds and the Mottled 

 Beds is not commonly well exposed in the Corris area ; but it may 

 be observed in rSTant y Goedwig and in the unnamed stream which 

 runs past the Vicarage, south of Corris Village. In both these 

 localities, and at several places south-westwards as far as Taren y 

 Gesail, the top of the Garnedd-wen Beds consists of a prominent 

 grit-band about 10 to 12 inches thick : as, for example, at Cwm 

 Caelum and in the old level on Taren Cadian. The rocks im- 

 mediately below this grit-band often consist of alternating thin 

 gritty and dark mudstone layers with a characteristic undulating 

 bedding, which imparts to them a ' wavy ' appearance. 



The junction of Ordovician and Silurian may be seen within a 

 foot or so in several places north-east of Corris, as, for example, 

 in the Afon Dulas, where it is joined by the Afon Neiriau (east 

 of Aberllefenni), etc. This locality is one where the actual 

 contact between the Mottled Beds and the Garnedd-wen Beds 

 might be examined in detail under favourable conditions. 



In the small unnamed stream south of Corris, mentioned above, 

 there is exposed a thin dark shale-band with a pronounced rusty 

 weathering, about 1 inch thick, and about 2 feet above the base 

 of the Mottled Beds. This thin band exhibits the peculiar litho- 

 logical characters of the band described at Machynlleth, at the 

 same horizon. It breaks up into thin brittle pieces, and is very 

 much weathered etc. In this locality at Corris it has yielded : — 



Glyptograptits perscnlptus Salter. 

 Mesogra/ptus modestus var. parvulus 

 (H. Lapworth). 



'Climacograptus scalaris var. 



miserabilis Elles & Wood. 

 Climacograptus scalaris var. 



normalis Lapworth. 



