part 1] IGNEOUS AND ASSOCIATED EOCKS OF LLANWRTYD. 23 



species as common or reaching their maximum degree of abundance 

 in the Mydrim Limestone. 1 The Llanwrtyd fauna includes no 

 less than five of these species. Moreover, only one of the Llanwrtyd 

 species {Amplexograptus perexcavatus, of which but one specimen 

 was discovered) is found at all commonly at any other horizon in 

 South Wales. We are, therefore, led to the conclusion that the 

 graptolitic horizons are homotaxial with the Mydrim 

 Limestone. This conclusion is further borne out, as we have 

 mentioned above, by the occurrence of ashy limestones. Since the 

 spilites and the Upper Ashes occur between the two graptolitic 

 horizons, it follows that they are of the age of the Mydrim Lime- 

 stone ; while the Lower Ashes, if not of the same age, cannot be 

 much earlier. 



For the sake of convenience, the Geological Survey memoir has 

 included the Mydrim Limestone and the Mydrim Shales in the 

 Bala (Caradocian) ; but the remark is made therein that the true 

 junction between the Llandeilian (Glenkiln) and the Caradocian 

 (Hartfell) should be drawn somewhere in the Mydrim Shales. 3 

 Prof. W. W. Watts draws the boundary between the Mydrim 

 Limestone and the Mydrim Shales, including the former in the 

 Llandeilian. 3 According to the Geological Survey classification, 

 the Llanwrtyd igneous rocks come at the base of the Cara- 

 docian ; according to Prof. Watts's classification, at the top of the 

 Llandeilian ; and according to Miss Elles's classification, 4 towards 

 the upper part of the Llandeilian (Glenkiln). 



(/) The Black Slates. 



The Upper Ashes are succeeded by a huge thickness of mono- 

 tonous dark slates, much contorted and cleaved. These beds form 

 the hills all round the igneous mass. A considerable search has 

 resulted only in the discovery of indeterminable fragments of 

 graptolites of Diplograptid type. Some grit-bands occur in the 

 slates at certain horizons. 



(g) The Intrusion. 



The main anticline is bounded on the western side by a great 

 fault, which must have a downthrow to the west, since it cuts off 

 part of the Upper Ashes. Along the line of this fault, and 

 apparently on its western side, there is a curious intrusion. 

 Unfortunately, it has not been possible to determine the exact field 

 relationships of this rock ; it may be intruded into the Black 

 Slates at an unknown distance above the Upper Ashes, or it may 



1 Mem. Geol. Surv. 1914, pp. 38-39. 



2 ' The Geology of the South Wales Coalfield, pt. x : The Country around 

 Carmarthen ' Mem. Geol. Surv. 1909, p. 46. 



3 ' Handbuch der Regionalen Geologie, vol. iii, pt. 1 : The British Isles ' 

 1917, pp. 72-73. 



4 ' The Relation of the Ordovician & Silurian Rocks of Conway (North 

 Wales) ' Q. J. G. S. vol. lxv (1909) p. 169. 



