SECTIONAL TRANSACTIONS.— C. 351 



limited by the Knowsley Anticline, the detached Skelmersdale Basin lying 

 to the north of it. 



It has been found impossible to analyse some of the folds and associated 

 faults. It can be shown that folding, but not faulting, took place before the 

 Upper Coal Measures were deposited which here rest unconformably on 

 the Middle Coal Measures, over 2,000 ft. of which are missing. 



The parallelism between the unconformable bases of the Upper Coal 

 Measures and the Permo-Trias suggests that there was little folding prior to 

 the deposition of the Permo-Trias although faulting took place. The major 

 faulting occurred in post-Trias times and was mainly a renewal of movement 

 along pre-Triassic faults. The relatively small dips in the Upper Coal 

 Measures and Trias compared with the Middle Coal Measures beneath 

 indicate that the folding was mainly pre-Upper Coal Measures. 



There are indications of movement during the deposition of the Middle 

 Coal Measures. 



Dr. G. H. Mitchell. — The Skipton anticline. 



Afternoon. 



Excursion to Bispham and Upholland. Leaders : Messrs. R. W. 

 Chalmers and L. H. Tonks. 



Tuesday, September 15. 



Prof. H. P. Lewis. — Ordovician succession at S.W. end of Aran range, 

 Merionethshire ( 1 o . o) . 



The area is that drained by the Helygog and Celynog, tributaries of the 

 Afon Wnion. It consists of Upper Cambrian and Ordovician rocks which 

 dip, in general, to the south-east, and which are crossed by the Bala Fault. 

 The Ordovician sequence, with approximate maximum thicknesses of the 

 subdivisions, is as follows : 



Arenig or Basement Series (900 ft.). — Gritty and micaceous flags, slaty 

 tuff, bedded and massive felspathic tuff and, at or near the top, the ' Aran 

 Boulder Bed,' in which the tuff contains abundant large boulders of ' felsite.' 

 South-east of the Bala Fault, the lowest beds are separated by a sill of 

 dolerite from grey flags of Tremadoc age (with Asaphellus homfrayi), in the 

 Celynog section, and faulted against Tremadoc rocks containing Dictyonema 

 and Acrotreta in the Helygog. 



Llanvirn Madstones and Flags (350 ft.). — These follow the Basement 

 Series with apparent conformity and consist largely of dark-blue, pyritous 

 mudstones, but in the lower part, in particular, bands of fine siltstone and 

 occasional thin ash-bands, or scattered fragments of pyroclastic felspathic 

 material, occur. The mudstones have yielded Calymene sp., Cryptograptus 

 tricornis (?) and graptolites of the Didymograptus bifidus group including 

 D. c.f. artus. These mudstones pass up into tough, blocky, blue mud- 

 stones with dendroid graptolites, and, south of the Bala Fault, into 

 micaceous and pyritic flags in which crinoids, Ogyginus cf. corndensis and 

 Trinucleus occur in addition, thus lithologically and faunally suggesting 

 equivalence with the Upper Llanvirn of western Shropshire. 



The Main Pyroclastic Group (1,800 ft.). Llandeilo-Caradoc (?). — Most 

 fully represented west of the Celynog, where it is subdivided into three 



