328 Geological Society. 



\ 



10. Hard light-grey siliceous flags. 

 LudlOW \ 9. Micaceous sandy shales, with rotten lime- 

 stone bands. 

 ( 8. Calcareous flagstones, with fossiliferous bands. 

 (/)7. Calcareous fissile shales, with beds of flagstone. 

 =Zone of Cyrtograptus Lundgreni. 

 (e) 6. Hard grey calcareous shales with graptolites. 



=Zone of Cyrtograptus riyidus. Feet. 



5. Light-coloured flags, unfossilifero us (400?) 



Wenlock (&) 4. Grey calcareous flags and shales, with Iiine- 



ShaleS i stone-concretions 300 



— Zone of Cyrtograptus Linnarssoni. 



(c) 3. Soft shales with harder flaggy beds 200 



= Zone of Cyrtograptus, sp. no v. 



(b) 2. Hard calcareous flagstones 300 



= Zone of Monograptus riccartonensis, 

 (a) 1. Soft fissile shales alternating with flags. 



=Zone of Cyrtograptus Murckisoni. 

 The Wenlock Shale of this area was deposited on the sinking 

 shore-line of the old Llandeilo ridge, resulting in the overlap of 

 higher beds on lower. 



The Long Mountain is a syncline with a north-east and south-west 

 axis. Here also there must have been deposition on a sinking ridge 

 accompanied by overlap. The lowest beds, exposed near Chirbury, 

 belong to the zone of Monograptus riccartonengis ; but on the 

 north-west side, near Middletown, the lowest beds seen above the 

 Tarannon Shales belong to the zone of Cyrtograptus Linnarssoni. 

 The relationship of these beds to the Tarannon Shales is regarded 

 as an unconformity. The sections in this district confirm those 

 established for the Builth area. 



In the Llangollen Basin the general structure is a synclinal fold 

 complicated by many minor folds and faults. The ' Pale Slates ' 

 are covered by darker shales belonging to the zone of Cyrtograptus 

 Murckisoni; and in this district the relationships of the lowest zones 

 are clearer than at Builth or the Long Mountain. 



The palseontological part of the paper describes several species, 

 some of which are recorded for the first time in the country : 

 Vyrtogrtrptus Carrutnersi, C. rigidity and a new species of Cyrto- 

 graptus of zonal value, four varieties of Monograptus FUmingii, 

 three of M. vomerinus, one variety of M. testis, and two new species 

 of Monograptus are among the forms dealt with. 



3. ' On an Intrusion of Diabase into Permo- Carboniferous Eocks 

 at Frederick Henry Bay (Tasmania).' ByT. Stephens, Esq., M.A., 

 F.G.S. 



The relationship of the abundant diabase to the Permo-Carboni- 

 feroas strata of the island has been long a matter of dispute. 

 Among others, Jukes described sections which appeared to confirm 

 the view that Permo-Carboniferous sediments were deposited round 

 vast masses of igneous rock previously cooled and denuded. The 

 author has identified and visited the sections, and finds in one that, 

 although there is a step-like junction between the sediments and the 

 igneous rocks, it is the result of the intrusion of diabase, and not 

 of the deposition of sediment. The sediment, which is fossiliferous, 

 is converted into an intensely hard whitish marble, and the associated 

 shale-bands into chert. The diabase, which is ordinarily an ophitic 

 rock, acquires at the junction a finely crystalline-granular texture, 

 .luke's's second section also gives undoubted evidence of intrusion. 



