226 MESSES. T. C. CANTEILL AND H. H. THOMAS ON THE [May I906, 



III. The Sedimentaey Hocks. 



The sedimentary rocks associated with the various igneous masses 

 consist of the following, in descending order : — 



j r ( Red marls and sandstones ; with a group of green 



n t> a „„ i marls and cornstones, sandstones, and conglome- 



Old Red Sandstone. ^ rateB at the baae. 



/ Bidymograptus-bifidus Beds. — Blue-black 

 shales with one or more thick bands of grit 

 (BifidiLS-grits) towards the base. Graptolites of 

 B. -bifidus type characteristic. 

 Tetragraptus-TSeds. — Black and buff shales, often 

 Ordovician / iridescent and iron-stained, with interbedded thin 



(Arenig). ] grit-bands. Thick bands of ashy grit and con- 



glomerate (Tetragraptus-grits) towards the base. 

 Fossils rare; the shales yield extensiform and 

 dendroid graptolites, Tetragraptus, and some horny 

 brachiopods; Bidymograptus bifidus is absent. The 

 grits yield occasional specimens of Orthis. 



(1) The Tetragraptus-l$e&s. 



The beds belonging to this subdivision crop out on some elevated 

 ground extending north-eastward from the village of Llangynog 

 towards the River Towy, and occupy an area some 4 miles long by 

 1| broad. They are faulted on the north against an outcrop of 

 Didymograptus -bifidus Beds, and on the south are bounded by the 

 Old Red Sandstone. Stretching across the middle of this shale- 

 area is a series of elliptical hills and ridges made up of grits and 

 conglomerates. The ridges, which are frequently bounded on all 

 sides by faults, are disposed in a zigzag line running from west 

 to east, from Pen-y-Moelfre and Moelfre Wood, through the Wind- 

 mill Hill, to Glog, a distance of 2 miles. Inasmuch as the grits 

 and conglomerates forming these ridges are more or less vertical, 

 the true order of succession is not obvious ; but we have every 

 reason to think that the grits and conglomerates are the same 

 as those which form the core of a dome-like anticline at Bola- 

 haul and Allt-cystanog south-east of Caermarthen, 1 where they 

 underlie the equivalents of the Tetragrajrtus-sh&les and constitute 

 the lowest member of the Arenig Series. At Llangynog they are 

 again brought up in the form of anticlines, and are the oldest part 

 of the Tetragraptus-Jieds. 



(a) Shales north of the Grits. 



On the north of the grits the best sections are to be found 

 along the upper parts of Nant Crymlyn, a stream which rises on 

 some peaty ground north of the village of Llangynog, and flows 

 north-westward to the Cywyn at Banc-y-felin. In this brook, due 

 north of Lan-y-gors, greyish-black splintery shales and mudstones, 



1 ' Summary of Progress of the Geological Survey for 1904 ' Mem. Geol. 

 £urv. 1905, pp. 33, 34. 



