THE LOWER SILURIAN PERIOD. 93 



3. The Caradoc or Bala Group. The alternative names of 

 this group are also of local origin, and are derived, the one 

 from Caer Caradoc in Shropshire, the other from Bala in Wales, 

 strata of this age occurring in both localities. The series is 

 divided into a lower and upper group, the latter chiefly com- 

 posed of shales and flags, and the former of sandstones and 

 shales, together with the important and interesting calcareous 

 band known as the " Bala Limestone.'' The thickness of the 

 entire series varies from 4000 to as much as 12,000 feet, ac- 

 cording as it contains more or less of interstratified igneous 

 rocks. 



4. The Llandovery Group (Lower Llandovery of Murchison). 

 This series, as developed near the town of Llandovery, in 

 Caermarthenshire, consists of less than 1000 feet of conglom- 

 erates, sandstones, and shales. It is probable, however, that 

 the little calcareous band known as the " Hirnant Limestone," 

 together with certain pale-coloured slates which lie above the 

 Bala Limestone, though usually referred to the Caradoc series, 

 should in reality be regarded as belonging to the Llandovery 

 group. 



The general succession of the Lower Silurian strata of Wales 

 and its borders, attaining a maximum thickness (along with 

 contemporaneous igneous matter) of nearly 30,000 feet, is 

 diagramatically represented in the annexed sketch-section 

 (fig. 34) : . 



[GENERALISED SECTION 



