18460 SEDGWICK ON THE FOSSIL SLATES OF N.WALES, ETC. 14<9 



mountain masses, more or less pyramidal in outline, and are of a 

 date posterior to that of the beds they are associated with — such are 

 the crests of Carn Goch, the Rivals, Carn Boduan, &c. &c. 



(3.) Great beds of contemporaneous trappean rocks (greenstones, 

 felstones, trap shales and trap breccias, &c.), striking and alternating 

 with, and passing into, slate rocks with organic remains, and occasion- 

 ally containing, among their own members, perfect traces of organic 

 remains. These rocks are best developed between Pwllheli and 

 Boduan, near both which places organic remains are numerous. 



The oldest stratified rocks of the fossiliferous series are (I think) 

 in the promontory between Hell's Mouth and St. Tudwal's road. 

 These I would compare (provisionally) with the rocks south of Tre- 

 madoc. Above them I would place (provisionally, for I have not 

 seen the sections since 1831) some slaty beds, west of Abererch, 

 which contain the pisolitic iron ore. The fossil-bearing rocks near 

 Boduan may, I think, be safely identified with the principal fossil bands 

 of the Snowdonian crest, viz. the bands where the genus Orthis 

 first appears in force. 



Lastly, the fossil bands alternating with contemporaneous por- 

 phyry and trappean shale, &c., W. and S.W. of Pwllheli — similar 

 bands half a mile west of Llanbedrog — and the slate rocks three or 

 four miles north of Pwllheli (on the Clynnog road, near Castell Gro- 

 gan) — appear to belong to a higher fossil group, and may perhaps 

 represent one of the lower fossil bands east of Great Arrenig. 



The following is a list of some of the fossils from the places above 

 named : — 

 (a.) Boduan Fossils. — Orthis flabellukim + -f . 



grandis (?) "| 



elegantula I -f &c. of a former list, 



Leptaena sericea J 

 (b.) Castell Grogan. — Homalonotus bisulcatus, MSS. 

 Encrinite stems. 

 Fucoids. 

 (c.) Llanbedrog and Pwllheli in one table, as follows : — 



Homalonotus bisulcatus^MS. 

 Phacops apieulatus, MS. ... 

 Calymene Blumenbachii ... 



Trinucleus Caractaci 



Euoraphalus (imperfect) ... 

 Pterocardia* (Bala sp.) ... 



Leptsena tenuistriata 



sericea 



Orthis calligramma 



expansa 



elegantula 



Spirifer radiatus 



Lynx 



Tentaculites annulatus 



Llanbedrog. 



+ 

 + + 



PwUheli. 



+ 

 + 

 + 

 + 

 + 











+ 







+ 



+ + 

 + + 



+ (m tt) 



+ 



To these add for Llanbedrog only, 

 Petropolitana {}) ; Ptilodictya dicJiotoma 



rasa ; F. {narrow branches) ; F, 

 aud Stromatopora concentrica. 



* A genus proposed in MSS. for species of Cypricardia, &c. of the Sil. Syst. 

 with elongate hinge teeth. ^»^" .*v*»i, #. ^^.>v, 



M 



VOL. III.^ — PART I. 



