1869.] On a Tertiary Geological Classification. 365 



the Lower Silurian series, of which the Bala and Llandeilo hme- 

 stone series form the Middle, and the Lower Llandovery rocks and 

 Caradoc sandstone, the upj^er sedimentary series. 



In Bohemia, however, as M. Barrande has shown, this group, 

 named by him " Zone primordiale," assumes a threefold arrange- 

 ment, with a central calcareous member, as represented in the 

 column of strata. 



Middle Silurian. — A Middle Silurian group is not yet gene- 

 raUy recognized, but, as Professor Eamsay has very clearly shown,* 

 there is a great physical break between the Lower and Upper Llan- 

 dovery beds, accompanied by the disappearance of the great majority 

 of species of animals belongmg to the Caradoc beds, while at the close 

 of the Upper Llandovery series (Tarannon shale) there is a second 

 break and unconformity. There are therefore apparently very good 

 grounds for a Middle Silurian group, of which the Upper Llan- 

 dovery limestone may be considered the central calcareous member. 



Upj^er Silurian Series. — The basement beds consisting of the 

 Derbyshire grits, flags, and slates, rest unconformal)ly on the Middle 

 Silurian series. This is the lower sedimentary member. The 

 middle calcareous member is represented by the Wenlock and 

 Aymestry limestone series. These two beds of limestone assume 

 in this coimtry an individuality and distinctive palseontological 

 character, which, as Sir Koderick Murchison has shown, is not 

 maintained by their Continental equivalents.! The Upper Ludlow 

 rock and tilestones combine to form the upper sedimentary member 

 of the group. 



Devonian Series. — This natural group appears under several 

 distinctive characters in difierent parts of the British Islands. 

 Nowhere, indeed, is the series represented imbroken and complete ; 

 but in Devonshire we have probably the nearest approach to this. 

 I shall therefore, in dealing with this great formation, call to my 

 aid those " breaks in succession " which Professor Kamsay has 

 recently dealt with in his Presidential address to the Geological 

 Society of London. | 



Devonshire. — The ternary division of the series into a lower 

 and upjjer sedimentary, with a middle calcareous member, seems to 

 have been very clearly estabhshed, both by the original observa- 

 tions of MM. Murchison and Sedgwick, and confirmed by the recent 

 detailed examination of Mr. Et]n-idge.§ The classification here 

 proposed differs slightly from that of these authors who have 

 established a Lower, Middle, and Upper Devonian series ; but as I 



* Presidential Address to the Geological Society of London, 1863. 



t This view is borne out by M. Barrande, who rehi's the tliree iijjjicr lime- 

 stones of tbe Sikirian series in Bohemia to the "Wenlock and Ludlow beds con- 

 jointly, fintling it impracticable to make the distinction of these beds into two 

 groups corresponding to the English series. 



X 1803-4. § ' Quart. Jourii. G'eol. Soc.,' vol. xxiii., p. 580. 



2 c 2 



