162 



WALTER KEEPING ON THE 



Denbighshire grits. We shall find reason to believe that they are. 

 represented, feebly it is true, by the thin grit-beds above described 

 in the Tarannon shale. 



The Dovey Valley : Section across the valley near Llany Mowddwy 

 (fig. 7). — This section gives, in a narrow area (measuring less 

 than 2 miles across), the complete series from the Lower Eala slates 

 to the Denbighshire grits. The Denbighshire grits pass down into 

 the Tarannon shales, which are pale-slate rocks resembling the 

 E-hyader pale slates. There is, again, no break between these and 

 the series of greywackes and dark shales and slates beneath. This 

 latter group (which is in part the Lower Llandovery of the Survey) 

 agrees both in its lithological characters and in its fossils, so far as 

 known, with the Aberystwyth grits. 



There is no distinct basement bed of conglomerate or grit to this 

 series, such as might indicate the existence of an important strati- 

 graphical break ; but, on the contrary, there is every appearance of 

 a passage from the underlying great slate group up into the true 

 Silurian series. This great slate group corresponds in its general 

 lithological characters and its " cone-in-cone " nodules with our 

 Metalliferous slates ; but I have not detected any pale slate in them. 

 In the absence of fossils, it is not possible to determine their exact 

 age ; but they should probably be regarded as belonging for the most 

 part to the Upper Bala slate group. I should, however, expect 

 some of its upper beds to be the representatives of our Cardiganshire 

 Group, the lower part (Upper Bala) corresponding with much of the 

 slate group between Cardigan and Llangrannog, about to be de- 

 scribed. 



It is further noteworthy that in this line of section, as in the 

 Cardiganshire Group, the newer rocks are much better cleaved, and 

 appear altogether more highly metamorphosed than the more ancient 

 rocks to the west. 



Corris Area. — The great slate district of Corris, usually referred 

 to the Llandeilo age, belongs to our Metalliferous -slate group, as is 

 proved by the following fossils from Corris and Taren y Gesail : — 



Monograptu3 Sedgewickii, PortL? 

 tenuis, Portl.? 



Climacograpsus scalaris, His. 

 Ortkoceras, sp. (same as the Cefn- 

 Hendre shell). 



The Cardigan District. — The pale felspathic grits and black slates 

 of Newport Bay and Cardigan, hitherto placed in the Lower Llan- 

 dovery series " b 4" are not of this age, but belong to the Middle Bala 

 or Caradoc Group ; as to this the fossil evidence is conclusive. Above 

 these come rolling beds of pale slates and shales, then darker shaly 

 slates, with a zone of pale felspathic grits ; these also I refer to 

 the Bala period. The overlying shaly slates and rab are passage- 

 beds of Caradoc-Llandovery age, presenting the gradual incoming 

 of the Aberystwyth grits. 



Eeviewing the several rock-groups and their distribution , we find : — 

 (A) The Plynlimmon grits, seen upon Plynlimmon and around 

 Rhyader, also probably in the hills west of Tregaron and Lampeter j 



