part 4] GEOLOGY OP CORHIS AND ABERLLEEENNI. 527 



all other rocks in the area, except possibly the Red Vein of the 

 Upper Ordovician. From the latter, it may be distinguished 

 lithologically by its more shaly character, rather more pronounced 

 rusty weathering, and absence of a coarse mottling which occurs in 

 parts of the Red Vein. In the Corns area the rocks of the Cwmere 

 Group are very highly cleaved, and graptolites are extremely 

 difficult to extract in a state capable of identification. 



The base of the Cwmere Croup is marked by the zone of 

 Olyptograptus persculptus, or, as it has been called, the 'Mottled 

 Beds '. This zone maintains all the characters which have been 

 described in the area farther south, and the band has been, in the 

 Corris-Aberllefenni area, as throughout much of Central Wales, a 

 valuable guide in deciphering the geological structure. For that 

 reason, it has been mapped right through from Machynlleth into 

 the Corris area, and for some miles north-east and east of the area 

 at present described. 



The zone consists of compact dark-blue mudstones, in which 

 paler bands occur. These paler bands are characterized by a 

 small irregular mottling. It has already been observed that the 

 Abercwmeiddaw Mudstones and part of the Red Vein are mottled ; 

 but the character of the mottling is quite different, and there is 

 no difficulty in distinguishing even small hand-specimens. The 

 mottling in the case of the Ordovician rocks is much coarser, the 

 blotches are much larger, and they are not confined to paler 

 bands Avithin a dark mudstone group. The actual character of the 

 mudstone and the Aveathering tints are distinct in each case. 



Throughout the area, the zone of Glt/ptograptus persculptus 

 rests with apparent conformity on the Garnedd-wen Beds. There 

 is an abrupt change in lithology at the junction, and it is this 

 lithological change which makes the zone so valuable from the 

 point of view of surveying. 



In describing the Machynlleth area, Prof. O. T. Jones and I 

 made the following statement : — 



' It is significant also that, in contrast with the constancy of character 

 which distinguishes the Mottled Beds, the strata upon which they rest exhibit 

 considerable variation from locality to locality. In the absence of fossili- 

 ferous horizons,' it is not possible to decide whether this is due to the vari- 

 ability of the underlying group, or to an overstep of the Mottled Beds on to 

 different horizons of the older beds ' (Q. J. G-. S. vol. lxxi, 1915-16, p. 351). 



Since that statement was made, the Mottled Beds, together with 

 the next underlying fossiliferous horizon : namely, the lied A r ein, 

 with Dicelloqraptus anceps and Ortlioc/ rapt us Iruncatiis var. 

 abhreviatus, have been mapped over a considerable part of the 

 area from Machynlleth through Corris to Dinas Mawddwy, a 

 distance of some 15 to 1G miles. The top of the Red Vein passes 

 into the Narrow A r ein, which is another datum-line capable of 

 accurate mapping, and has been traced continuously from west of 

 Corris to Dinas Mawddwy. 



The strata upon which the Mottled Beds rest do undoubtedly 

 exhibit variation from locality to localit}' ; but the Mottled Beds 

 do not overstep on to different horizons of the older beds. The 



