TllK OHDOVlriAs BTBTIM lL'3 



When traced westward, however, this grit thins out and its 

 plan; is taken by a band of volcanic ash, while tin- upper part of 

 tin- flagstone series passes into black shale, the whole at the same 

 time greatly diminishing in thickness, so that near Carmarthen the 

 group is less than 900 feet thick and the succession is : 



Fwt 

 II mire shales with Dicellograptns divaricatus . . . 500 



Llandilo flags and limestones about 2.10 



Volcanic ash with Asaphus tyrannus ..... about 70 



Still farther west the "Asaphus ash " also thins out, and the 

 flaggy limestones are condensed into one band of dark grey or 

 black limestone which rests directly on the Llanvirnian shales. 



On the north side of the St. David's anticline the series has again 

 become much thicker and is divisible into two stages as below: 



Feet. 

 Black slates and flags with Ogygia Buchi and Calymene 



duplicate/, 1000 



Calcareous flags and limestone with Asaphus tyrannus and 



Calymene cambrensis about 800 



The restricted Llandilo Series has not yet been mapped either 

 in Merioneth or Carnarvonshire. The upper andesitic and 

 rhyolitic ashes of the Arenig Mountains are regarded by Mr. 

 Fearnsides as belonging to this series, and they are unconformably 

 overlain by a fossiliferous limestone which seems to belong to the 

 Bala Series. In the anticline of the Berwyn Mountains to the 

 eastward Llandilo Beds appear again with a more normal facies, 

 having a band of limestones at or near the base which are about 

 400 feet thick and are full of such fossils as Asaphus tyrannus, 

 Calymene cambrensis, Orthis turgida, etc. 



In the Snowdonian district there is a great development of 

 volcanic rocks which were included by Ramsay in the Bala Series, 

 but are now thought to be largely of Llandilo age ; for upon the 

 highest volcanic rocks of Snowdon rest black shales crowded with 

 species of Diplograptus which seem to indicate a low horizon in the 

 Bala Series. 



The lavas and ashes of the Snowdon range seem to have issued 

 from a group of volcanic vents which probably formed volcanic 

 islands in the Ordovician sea. Near the actual centres of eruption 

 both ashes and lavas are found, the latter being usually rhyolites, 

 but in other localities, presumably more distant from the volcanic 

 vents, ashes are the prevalent deposit, and these are interstratified 

 with some bands of slaty shale. This volcanic series extends to 

 the very northern extremity of Carnarvonshire. 



