580 



PROP. W. J. PUCill OX THE GEOLOGY OF THE [vol. lxxix. 



Derwen Group. — As at Machynlleth, the rusty- weathering 

 shales of the Cwmere Group are overlain by a characteristic group 

 of pale-grey mudstones, with comparatively thin dark shale-hands 

 which yield graptolites. The change in lithology takes place at 

 the base of the zone of Mesograptus magmcs, and is one of 

 considerable assistance in surveying. This boundary has been 

 mapped throughout the Corris area. 



The Derwen Group has not been examined in detail in the 

 Corris area ; but it possesses the same lithologieal characters as in 

 the area farther south, and there is little doubt that, if investigated 

 in detail, it would reveal the same faunal succession. 



From the Derwen Beds at Taren y Gesail, between the two sets 

 of disused quarries, the following graptolites have been obtained : — 



Climacograptus hughesi (Nicholson). 

 Climacograptus scalaris (Hisinger). 

 Climacograptus scalaris var. nor- 



malis Lap worth. 

 Petalograptus minor Elles. 

 Petalograptus palmeus var. tennis 



(Barrande). 

 Monograptus convolutus (Hisinger). 



The zone is that of Monograptus regular is, as defined at 

 Machynlleth. 



From Nant y Goedwig the following were obtained: — 



Monograptus decipiens Toraquist. 



M. gemmatus (?) (Barrande). 



M. lobifervs (M'Coy). 



M. regularise?) Tornquist. 



M. tenuis (Portlock). 



Monograptus sp. 



Rastrites Jiybridus (Lapworth). 



Climacograptus hughesi (Nicholson). 

 Climacograptus scalaris (Hisinger). 

 Climacograptus scalaris var. nor- 



malis Lapworth. 

 Orthograptus bellulus Tornquist. 

 Monograptus argutus (?) Lapworth. 



Monograptus clingani (Carruthers). 

 M. convolutus (Hisinger). 

 M. decipiens Tornquist. 

 Petalograptus palmeus var. lotus 



(Barrande). 

 Rastrites peregrinus (Barrande). 



From Ffridd Newydd, south of Aberllefenni, the following 

 were obtained : — 



Climacograptus hughesi (Nicholson). 

 Climacograptus scalaris (?) (Hisinger). 

 Petalograptus minor Elles. 

 Monograptus argutus (?) Lapworth. 

 M. convolutus (Hisinger). 



Monograptus decipiens Tornquist. 

 M. lo'biferus (M'Coy). 

 M. regularis Tornquist. 

 M. tenuis (Portlock). 

 Rastrites hijbridus (Lapworth). 



From Nant Cwm Cadian the following were obtained : — 



Tornquist. 



Monograptus convolutus (Hisinger). 

 M lo'biferus (M'Coy). 



Monograptus regulai 

 Climacograptus sp. 



In the same stream-section, a short distance above the Ilono- 

 graptus-coiivolutus Beds, the following graptolites were col- 

 lected : — 



Monograptus halli (Barrande). 

 M. sedgivicki (Portlock). 



Monograptus tenuis (Portlock). 

 Climacograptus scalaris (Hisinger). 



The total thickness of the Derwen 

 200 feet. 



Beds is rather less than 



