﻿186 ME, W. Gr. TEA.ENSIDES ON THE [May I9IO, 



specimens of a variety of Angelina. They are the ' Middle Tre- 

 anadoc ' of Salter and the Sliumardia Shales of Arenig. 



The Garth Hill Beds are finely-bedded grey-blue slates with 

 some flaggy bands, and are always very rusty. They are the ' Upper 

 Tremadoc ' of Salter, and are characterized by the abundance of 

 Angelina} There is no evidence upon which to correlate them 

 with any particular formation in South Walesi and they are 

 certainly cut out by the Ordovician unconformity at Arenig. 



The Arenig Grit of Ynys-towyn is 70 feet thick. It is placed 

 in the Arenig by analogy only ; but near Criccieth a few Arenig 

 fossils have been found in ashy beds which overlie a similar deposit. 

 The strata above the ashy beds with Arenig fossils at Criccieth are 

 slates, so cleaved and crushed that the collecting of fossils is 

 impossible, and the local distinction between Arenig and Llandeilo 

 slates remains unknown. In the eastern district the Penmorfa 

 Fault has cut out the Arenig beds. 



The Llandeilo Beds of the east are dark slates, much crushed 

 and broken into slickensided augen by the faulting. Prom certain 

 of their augen in which cleavage has been destroyed and bedding 

 partly revived by the metamorphism due to a later dolerite, the 

 graptolites of Tyddyn-dicwm, Ynys-galch, and Pen-syflog have been 

 collected. These all belong to the zone of Nemagraptus gracilis or 

 to the overlying subzone of Climacograptus peltifer, and are of the 

 horizon of the Glenkiln Beds of Moffat, 2 or of the shales immediately 

 above the Mydrim Limestone in South Wales (Llandeilo). 3 



The higher grey slates are everywhere crushed, cleaved, or baked 

 by the dolerites, and have yielded no fossils which determine their 

 horizon. The andesites of the Tremadoc Ynys and the related 

 andesitic ashes and agglomerates of the Penmorfa flag-quarry, 

 Y Glog, and several localities, are interstratified among these grey 

 slates. They thin out south-westwards and thicken eastwards, and 

 may be attributed to some volcanic source upon the Moelwyn. 

 The felsites of the Criccieth district are intrusive, but belong 

 to the Snowdonian stage of volcanic activity. The intrusion of 

 the gabbroid dolerites 4 of Moel-y-gest, Y Gesail, Tremadoc, and 

 Pant Ifan is of some date more recent than the impression of the 

 •cleavage of the country. 



EXPLANATION OF PLATES XV-XVII. 

 Plate XV. 



Sections showing the relations of Cambrian and Ordovician rocks across the 

 Penmorfa Fault, on the scale of 6 inches to the mile : — A, from Morfa- 

 bychan to Fach Goch ; B, from Glan-byl to Pen-yr-allt. 



1 H. Hicks, Quart. Journ. Geol. Soc. vol. xxix (1873) p. 43 & \ol. xxxi (1875) 

 p. 175. 



2 C. Lapworth, ibid. vol. xxxiv (1878) p. 253. 



3 Summary of Progress of Geol. Surv. 1906, p. 44. See also ' Geology of the 

 Country around Caermarthen ' Mem. Geol. Surv. 1909, p. 49. 



4 A. Harker, 'Bala Volcanic Series of Caernarvonshire' (Sedgwick Prize 

 Essay) 1889, pp. 76-79. . 



