Vol. 52.] LLANDOVERY AND ASSOCIATED ROCKS OF CONWAY. 275 



The geological structure of the area is fairly simple, though the 

 beds are faulted and give indications of having been disturbed, the 

 cleavage in many cases being in a direction at right angles to that 

 of the plane of bedding. There is no great variation in the litho- 

 logical characters of the beds ; they are all of the general type 

 known as 'greywacke,' and graduate locally from thick-bedded 

 gritstones, through smooth flagstones, and thin-bedded hard grey 

 shales down to fine-grained graptolitic mudstones. The only fossils 

 with which we have to deal are graptolites, and their value for 

 purposes of stratigraphical correlation is now too well known to 

 need comment. 



II. Literature. 



The succession of rocks at Conway has received but little attention 

 at the hands of geologists, and the literature is in consequence 

 somewhat scanty. In the second edition of the Geological Survey 

 Memoir on North. Wales (1881) the succession at Conway is given 

 as follows : — 



Wenlock Shale, 1 Wenlock Beds . 

 Denbighshire Grit. J 

 Tarannon Shale. 



( Upper and Lower Llandovery Beds absent.) 

 Caradoc or Bala Beds. 



It is also suggested that the absence of Llandovery rocks between 

 Conway and the country east of Bala Lake may possibly be due to the 

 overlap of the Tarannon Shales, which are said to rest on Lower 

 Silurian rocks at about the horizon of the Bala Limestone. 



The Tarannon Shales form a narrow band at the base of the 

 Denbighshire Grits extending from Conway to Llanbedr, about 5 

 miles to the south, being interrupted only by three small faults 

 with a downthrow to the east. 



On the hill immediately south of Conway the base of the grits, 

 resting on a narrow band of Tarannon Shales, strikes towards 

 Gyffin and, dipping east at an angle between 10° and 30°, passes 

 south by Y-Ro to Caerhun, where the western boundary is lost 

 in the alluvium of the liiver Conway. It is also stated that the 

 grits pass up into the Wenlock Shale, but no mention is made of 

 the considerable thickness of Wenlock Shale which we have found 

 below the lowest grit-band in this district. 



Prof. Lapworth, in his classical work on the 'Distribution of the 

 Ehabdophora ' (Ann. & Mag. Nat. Hist. ser. 5, vols. v. & vi. 1880, 

 pp. 45, etc.), records the occurrence of many graptolites in the 

 Tarannon Shales of Conway. On palseontological grounds he con- 

 cludes that the Tarannon Shales of Conway correspond to the lowest 

 portion of the Gala Group of Southern Scotland, that is, with the zone 

 of Monograptus exiguus. 



Our work necessitates a modification of the views of the earlier 

 Surveyors, though we are in complete agreement with Prof. Lap- 

 worth's suggestion as to the horizon of the Tarannon Shales of this 

 district. 



Q. J. G. S. No. 206. u 



