CHAPTER VII 



SILURIAN SYSTEM 



A. NOMENCLATURE AND DIVISIONS 



Nomenclature. The reasons for restricting the name Silurian 

 to the system called Upper Silurian by Murchison have been 

 given on p. 7 1, but no mention was there made of a subordinate 

 question, namely, the line of separation between the Ordovician 

 and Silurian. Originally the whole of the strata subsequently called 

 Llandovery Beds were included by Murchison in his Upper 

 Caradoc Group, but in 1854 Sedgwick and M'Coy proved that the 

 uppermost sandstones (to which they gave the name of May Hill 

 sandstone) had no connection physically or palzeontologically with 

 the Bala rocks, but formed the natural base of Murchison's (Upper) 

 Silurian Series in the typical Silurian areas. 



Kecognising the justice of this correction, Murchison then 

 separated the beds from the Caradoc sandstone (Bala), and created 

 a new group under the name of " Llandovery rocks," subdividing 

 it into two stages a lower (linked by some species to the Bala 

 Series) and an upper (closely connected with the Wenlock rocks), 

 but clearly united to one another by a community of Pentameri 

 and other fossils. This Llandovery Group he regarded as forming 

 a transition series between his Lower and Upper Silurian. 



Some geologists, however, objected to this arrangement on the 

 ground that there is frequently a physical break (overlap leading 

 to unconformity) between the Lower and Upper Llandovery, so 

 that the latter in such localities is dissociated altogether from t In- 

 former and from the Bala Beds. Consequently, Sedgwick, Jukes, 

 and others drew the line of separation at the base of the Upper 

 Llandovery or May Hill sandstone. But elsewhere there is little 

 or no want of conformity between the members of the Llandovery, 

 and it was pointed out in Chapter II. that small regard must be 

 paid to such local transgressions and unconformities, unless they 

 coincide with a palseontological break. 



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