336 PEOF. 0. T. JoxEs ox THE [Sept. 1912^ 



Between Conwil and Newcastle Emlyn is an extensive tract of 

 slaty rocks indicated on the Geological Survey map (Sheet 40) as 

 * argillaceous slates/ ' argillo-arenaceous slates/ ' argillaceous slates 

 with thin grit-bands' and occasional sandstones. As the rocks 

 around Newcastle Emlyn are of Ordovician age, while those of ConwiL 

 are at or near the base of the Silurian, the intervening slaty tract 

 must be synclinal in form and shared by rocks of the two systems. 

 Their line of demarcation is entirely unknown. At New Quay Road 

 Station are dark-blue cleaved mudstones resembling very closely the 

 higher Bala rocks of Cardiganshire ; while, between that place and 

 the next station (Maesycrugiau), are exposures of bright rusty- 

 weathering shales of Lower Birkhill type. It is suggested, there- 

 fore, that the Ordovician-Silurian boundary lies somewhere near 

 these localities. 



The only other district concerning which information is forth- 

 coming is the coast south of Llangranog. The Dicellograj^tus-hesiT'mg: 

 mudstones (p. 33-1:) are there succeeded by a considerable thickness 

 of grey grits ; these are followed by 200 to 300 feet of dark-blue, 

 flaggy, sandy shales and mudstones, which are exposed on the steep 

 slopes south of Traeth Bach (three-quarters of a mile south-west of 

 Llangranog). I have searched these shales for fossils, but without 

 success. They appear to be followed conformably by dark shales 

 with thin bands of dark-grey grits showing curious markings like 

 annelid trails, and closely resembling in their lithological characters 

 the beds at the base of the Aberystwyth Grits farther north. 

 On the Geological Survey map the Aberystwyth-Grit formation 

 reaches the coast at this point. 



Owing to the nature of the cliffs, I have been unable, so far, to 

 examine closely the junction of the Bala and the overlying grits ; 

 but there is little doubt that we reach here the junction between 

 the Bala and the Silurian rocks sought for by Keeping. As, however, 

 the base of the Aberystwyth Grits lies in this locality only a few 

 hundred feet above the base of the Silurian, whereas farther north, 

 the underlying beds are probably ten times as thick, some explan- 

 ation of their anomalous position is required. Several explanations 

 are possible, namely : — (] ) The Aberystwyth Grits may here be 

 equivalent palaeoutologically to the greater part of the Birkhill and 

 Gala rocks, whereas farther north they represent only a part of the 

 higher Gala rocks : that is, the grit facies commences at successively 

 lower horizons towards the south, as has been described among the 

 Ordovician and Silurian rocks of the South of Scotland. (2) There 

 may be an unconformity or overlap, either at the base of the grits 

 overlying the Bala, or at the base of the Aberystwyth Grits, or even 

 at both levels. The conditions would then be similar to those 

 among the Upper Birkhill and Gala rocks of Ehayader, as described 

 by Dr. H. Lapworth. This section requires further investigation, 

 but it is only at certain times that the state of the tide will admit of 

 its examination. 



The course of the base of the Silurian between this point and 

 New Quay Eoad is hypothetical, but the line drawn on the map. 



