190 DR. WILLS A^D ME. SMITH OX THE LOWER [vol. Ixxvili, 



Asligilliaii Seiies north of the Llangollen Synclinorium. 



Cyrn-y-Brain Beds and Plas-nchaf Grit. — The Ordo- 

 vician inliers of Cyrn-y-Bram and Mynydd-Cricor are formed of 

 greywacke-slates, sandstones, and grits, which can be correlated 

 by their abundant fauna with the Dolhir Beds and the Grlyn- 

 Corwen G-rit. Both inliers are anticlinal in structure, but the 

 lowest beds exposed occur on Cyrn-y-Brain. Those have been 

 detected north of Plas-uchaf, at one place only, and consist of soft 

 micaceous sandstones, slightly cleaved and fvill of fossils. Above 

 them follows a great thickness (probably 1500 feet at least) of 

 cleaved gritty greywacke-slates, which, especially towards the top, 

 become sandstones or grits. The Plas-uchaf Grit, which forms 

 the summit of the series, is a massive uncleaved rock, from 10 to 

 20 feet thick on Cyrn-y-Brain, and quite comparable with the 

 Corwen Grit. On C}T.'n-y- Brain its highest part is composed of a 

 tough platy sandstone with contorted lamination, about 3 feet 

 thick, passing upwards with appai'ent conformity into a dark 

 mudstone that weathers into a brown rottenstone. This may be 

 taken as the base of the Yalentian. On Mynydd-Cricor the thick- 

 ness of the Plas-uchaf Grit varies greatly, and it appears to split 

 and include bands of shale closely similar to the underlying cleaved 

 greywacke-slates. 



Throughout the Cyrn-y-Brain Beds, brachiopods are locally 

 plentiful. Of these, Jleristina crassa is the most characteristic 

 and abundant form, occurring from top to bottom of the series. 

 It was on account of the abundance of this fossil that the late 

 Prof . T. McKenny Hughes^ assigned the Plas-uchaf Grit (and pre- 

 sumably the underlying arenaceous beds) to the Llandovery Series. 

 Our collections, however, prove that this brachiopod occurs here 

 in association with undoubtedly Ordovician trilobites, such as 

 Trimicleus and Chasmops macroiira, the latter being of more 

 frequent occurrence than Trimicleus. 



The list of fossils given below (p. 191) shows that the Cyrn-y- 

 Brain Beds, together with the Plas-uchaf Grit, are the equivalent 

 of the Dolhir Beds proper and the Glyn-Corwen Grit. 



Prof. 0. T. Jones has pointed out to us that the types of 

 Flectambonites sericea and of StropJiomena antiquafa found in 

 the Cyrn-y-Brain Beds resemble those occurring in the Slade Beds 

 of South Wales. The discovery of these forms, in shales inter- 

 bedded with the Plas-uchaf Grit on Mynydd-Cricor, is useful 

 evidence in favour of the Ordovician rather than of the Silurian 

 age of that grit. 



Orfhis hirnantensis and Stropliomena siluriana are most 

 commonly found in, if not confined to, the upper part of the Cyrn- 

 y-Brain Beds. These forms, in conjunction with Orfhis sagittifera 

 and O. (Plati/strophia) hiforata, ysiY.^ssicostata, point to a close 

 comparison with the Hirnant Limestone and its associated rocks 



1 Q. J. G. S. vol. xxxiii (1877) p. 207. 



