Ch. xxvn.] 



LLANDEILO FLAGS. 



565 



occurs under the form of trappean tuff (volcanic ashes of De la 

 Beche), as in the crest of Snowdon, the peculiar species which dis- 

 tinguish it from the Llandeilo beds are still observable. The forma- 

 tion generally appears to be of shallow-water origin, and in that 

 respect is contrasted with the group next to be described. Professor 

 Rarnsay estimates the thickness of the Bala Beds, including the con- 

 temporaneous volcanic rocks, stratified and unstratified, as being from 

 10,000 to 12,000 feet in thickness. 



Llandeilo Flags. — The Lower Silurian strata were originally divided 

 by Sir R. Murchison into the upper group already described, under 

 the name of Caradoc Sandstone, and a lower one, called, from a town 

 in Caermarthenshire, the Llandeilo flags. The last-mentioned strata 

 consist of dark-colored micaceous flags, frequently calcareous, with a 

 great thickness of shales, generally black, below them. The same 

 beds are also seen at Builth in Radnorshire, and here they are inter- 

 stratified with volcanic matter. 



A still lower part of the Llandeilo rocks consists of a black car- 

 bonaceous slate of great thickness, frequently containing sulphate of 

 alumina, and sometimes, as in Dumfriesshire, beds of anthracite. It 

 has been conjectured that this carbonaceous matter may be due in 

 great measure to large quantities of imbedded animal remains, for 

 the number of Graptolites included in these slates was certainly very 

 great. I collected these same bodies in great numbers in Sweden and 

 Norway in 1835-6, both in the higher and lower graptolitic shales 

 of the Silurian system ; and was informed by Dr. Beck of Copen- 



Fig. 649. 



(Old plate, fig. 599, p. 442.) 



Fig. 650. 



Didymograpsus ( Graptolites) 



Murchisonii, Beck. 



Llandeilo flags. Wales. 



Diplograpsus pristis, 



Hi singer, sp. 



Shropshire ; Wales ; Sweden, &c. 



Llandeilo flags. 



Fig. 651. 



Fig. 652. 





Raslrites peregrinus, Barrandc. 



Scotland; Bohemia; Saxony: 



Llandeilo flags. 



Diplograpsus folium, Hisinger. 



Dumfriesshire; Sweden. 



Llandeilo flags. 



hagen that they were fossil zoophytes related to the Virgularia and 

 Pennatula, genera of which the living species now inhabit mud and 



