SEDGWICK ON NORTH WALES. 7 



The above calcareous rocks, which may be termed those of Glyn 

 Dyffws and Rhiwlas, might be supposed, from their proximity and 

 almost uniform strike, to belong to one deposit; but no proof of 

 such a connection is obtained by the evidence of sections, the in- 

 terval between the above two series of localities being much 

 disturbed and broken. Moreover, the fossils of the Rhiwlas beds, 

 considered as a whole, appear to differ from those of Grlyn Dyffws, 

 which agree with those of the limestone band, known by the name 

 of the " Bala limestone," on the eastern side of the lake. It is 

 clear that the Rhiwlas limestone lies far below that of Bala ; for 

 the strike of the former passes a mile to the west of the western 

 shore of the lake ; and in that line of strike calcareous beds are 

 found, though not in the form of limestone, agreeing, in respect 

 of their organic remains, with the Rhiwlas series. 



The fossils of Glyn Dyffws and Rhiwlas will be treated of in 

 describing the first line of section. 



Fossiliferous bands, which occasionally pass into limestone, are 

 also found at several places a little to the north-west of the locali- 

 ties which afford the Rhiwlas limestone. Those places are, 1. the 

 valley above Pentre Cwmda ; 2. a spot east of the mountain road 

 between G-arw fynydd and Moel Emoel ; and 3. Eglws Anne in 

 the forks of the Nant-y-Coegnant. Unless there be an inversion 

 of the beds in all that district (and of such an inversion the 

 author could perceive no indications) these last-mentioned calca- 

 reous bands must lie considerably below the Rhiwlas limestone.* 



§ 3. Transverse Sections across the Southern End of the Berwyn 



Chain.\ 



Section I. 

 Arenig Fawr to the Tanat River at Llangynog. 



Horizontal base 1 5 miles. 



W. 30° N. E. 30° s. 



Arenig Moel-v- Bala Rhiwae- Aher Llangy- 



Fawr. Garnedd. Lake. dog. Hirnant. nog. 



a b c d d' d" e f g h i it j 



v/^. 



* At a still lower level, apparently, lie the non-fossil iferous bands of lime- 

 stone, which occur at the following places : — 



1. To the S.W. of Arenig Fawr, in the upper branches of the Lliw. 



2. Near Hengwrt Uchaf, on the road from Dolgelly to Bala; the limestone 

 forming three bands, which were at one time worked for lime. 



3. On the east flank of Cader Idris. 



4. On the road from Dolgelly to Dinas Mowddy. 



These non-fossiliferous bands are all crystalline ; and appear to have been 

 much altered by igneous rocks. 



f The lines of section, with their numbers attached, are laid down on the 

 Map, which accompanies this Abstract. 



b 4 



