468 



Geological Society. 



In the detailed succession now worked out, the Ashgillian and the 

 Valentiau are for the first time distinguished, while the distinction 

 between the Wenlock and the Ludlow Beds is brought out by means 

 of graptolite zones. The detailed sequence is as follows : — 



r 



Salopian. 

 1000 feet. 



Ludlow 



Wenlock. < 



Yr Allt Group. 



B. Western facies. 



A. Eastern facies. 



| Buttington Group 

 ( = Tarannon Shales) 



B. Western facies 



Valenttan. 

 700 feet. 



AsHGILLIAN. 



50 feet. 



CARADOCIA.N. 



1300 feet, 



Gleskiln- 



Haktfell 



( = Dicrano- 



graptns 



Shales). 



U p p e r 

 Llandovery. 



Lower 

 Llandovery. 



(Cloddiau Group). 



A. Eastern facies 

 (Cefn Group). 



Gwern-y-Brain Beds 



Gaer-fawr Group. -{ 



LPwll-y-Glo Group. SI 



) 



J 



T r i 1 o b i t e 

 f DingleGroup. 1 1 



3. Sandy flags and shales, some- 

 times calcareous. 

 2. Hard thick flags with thin 

 shale-bands and septaria. 

 [_1. Thin muddj r shales. 

 C Blue flags, gritty, with calca- 

 \ reous concretions or boulders. 

 fBlue Hags, as a rule minutely 

 J false-bedded, with earthy mud- 

 'i stones, and a thin limestone- 

 L bed. 



Green and purple barren shales. 



2. Thick calcareous flags and 



mudstones. 

 1. Blue shales. 

 C Thick calcareous flags, with 

 \ Pentamerus Limestone. 

 TRed sandstone and conglo- 

 Powis Castle Group. ■{ merate, with occasional lime- 

 1^ stone developments, 

 f Black shivery shales, phosphatic, with a band 

 <t of black crystalline limestone near the base 

 [^ (= Staurocephalus Limestone ?). 



Calcareous mudstones and limestones (=Bala 

 Limestone) ; coarse, ashy, felspathic bands. 

 Thin, phosphatic, pale-grey shales at the base. 

 Massive grits, with bastard limestones, 

 Flags and grits, with some shale-bands, 

 ales and flags, with some grit-bands. 



Nodular mudstones and grey shales, with 



Trinticleus. 

 Splintery grey shales, with Diplograptus 



(Mesograptus) fcliaceus, etc. 



u 



The stratigraphical succession is shown by traverses ; but, owing 

 to successive overlap of the upper beds, no single traverse gives the 

 full succession. The different groups are traced over the area 

 mapped, and the chief fossiliferous exposures described, with full 

 lists of the fossils obtaiued. The district is shown to be transitional 

 iri character between neighbouring districts on almost every side, 

 combining many of their features. 



The structure of the area is mainly that of an anticline with 

 * keystone ' faulting, the dominant features being determined by 

 the intrusive masses of the Breidden Hills and the small Standard 

 Dyke of Welshpool. The two boundary-faults of the arch have 

 very considerable downthrows. 



A brief account is given of the Welshpool Dyke, with chemical 

 analyses of the rocks. 



The Glacial geology of the area is also described, three series of 

 deposits being observed: (1) a high-level series, (2) a low-level 

 series, and (3) a stratified series. The Guilsfield Valley is shown 

 to have been occupied by a Glacial lake, and the reversed drainage 

 of the Cefn-Yspn Brook is shown to be connected with a ' col ' 

 through which the overflow water drained. 



