8 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



1. Immediately to the west of Arenig Fawr slates occur (a), 

 dipping eastward. They contain Asaphus Buchii, and a few other 

 fossils. 



2. Next occur the Porphyries of Arenig (b), which are regarded 

 by the author as old eruptive or recomposed trappean rocks, of 

 cotemporaneous date with the slates with which they are associ- 

 ated. They form, therefore, no determinate base for the protozoic 

 rocks of North Wales. 



3. Upon the Porphyries rests a thick deposit of dark earthy 

 slates (c) dipping eastward, and extending in that direction about 

 a mile. Towards the upper limit of this bed numerous fossils 

 occur ; viz. Asaphus Powisii, Trinucleus Caractaci, Leptcena 

 sericea, Encrinital stems, &c. 



4. Immediately over the preceding is a still thicker bed of grey 

 slate (d, d\ d"\ which, including the very fossiliferous band, df, 

 supposed to be the equivalent of the Rhiwlas limestone, extends to 

 the western shore of Bala lake. Measured in a direction transverse 

 to the strike, the horizontal distance to the lake exceeds two miles ; 

 and as the dip, with one very limited exception, is steadily towards 

 the east, and at a very considerable angle, the thickness of this 

 bed must be great. The whole of the bed is fossiliferous. Near 

 Moel-y-Garnedd were found an Asterias, Orthis Jiabellulum, En- 

 crinital stems, &c. Further eastward are the very fossiliferous 

 slates, the supposed equivalent of the Rhiwlas limestone ; and 

 close to the margin of the lake, still higher, fossil bands appear. 



The total thickness of these fossiliferous beds west of Bala lake, 

 without including the masses of interbedded Porphyry, is estimated 

 by the author at not less than 2000 feet. 



[The series of the Rhiwlas limestone, and of the fossiliferous beds west of 

 Bala lake, is characterised by an abundance of Orthoceratites, and by Asaphus 

 Buchii, Illcenus Bowmanni (a new species), and other Trilobites. To these add 

 Asterias primceva. Notwithstanding the considerable number of species of Bra- 

 chiopoda contained in the list of fossils of the Protozoic rocks of North Wales 

 {vide List I.), the number of such remains in the Rhiwlas series is very small.] 



4. The breadth of the lake is supposed to be occupied by a group 

 of hard quartzose slates (e) ; since further to the south such slates 

 are seen to rest on the beds associated with the Rhiwlas limestone. 

 Their thickness- is not less than six or seven hundred feet. 



5. The first group on the east side of the lake consists of a se- 

 ries of hard grey slates (/), which contain some highly fossiliferous 

 bands. Some of these are much contorted on the line of strike ; 

 but their aggregate thickness is computed at not less than 500 feet. 



6. Next occurs the Bala limestone (g), a complex group about 

 100 feet thick, containing two bands of impure limestone, one only 

 of which, about 12 feet thick, is worked for lime. In one place it 

 contains a bed of schaalstein. 



[The Bala limestone and the Glyn Dyffws beds are marked by multitudes of 

 Orthides, particularly O. Actonice and O. Vespertilio, besides Leptcena tenuistriata, 

 and, in some places, an abundance of Asaphus tyrannus and A. Powisii. They 

 contain few species of coral, but specimens are very abundant, and these belong 



