340 SUPPOSED BREAKS IN SUCCESSION. 



Breaks in Succession of the British Strata. 



There are two kinds of breaks in succession : first, the physical 

 interruption in stratification, which is marked by unconformity ; 

 secondly, the palseontological break, which consists in a change in 

 fossils without any necessary variation in the rock sequence. The 

 latter condition can only claim to be a break in succession on the 

 hypothesis that the change in life has been brought about by upheaval 

 in some adjacent sea, which has caused the life to migrate away from 

 the upheaved region, and thus has disturbed the succession of life, in 

 an adjacent area which we may be examining, by causing an immigra- 

 tion which has changed its fauna. 



Breaks in the Primary Eocks. The nature of the break which 

 divides the Harlech and Bangor groups of Lower Cambrian rocks from 

 the unfossiliferous Pre-Cambrian is essentially a palasontological break 

 due to the absence of fossils from the lower rocks, though the older 

 beds are more highly volcanic. The fauna of these Lower Cambrian 

 rocks includes 18 genera and 32 species, of which one-third pass 

 into the Menevian. The Menevian beds again are defined in the 

 St. David's area by a palaeontological break. The fauna comprises 

 24 genera and 52 species, of which 19 pass into the Lower Lingula 

 flags. The Lower Lingula flags is a palseontological group with 17 

 genera and 36 species, chiefly Crustacea, of which 2 pass into the 

 Middle Lingula flags and 8 into the Upper Lingula flags. The Upper 

 Lingula flags contain 16 genera and 41 species, of which 10 pass into 

 the Lower Tremadoc and 7 into the Upper Tremadoc. Still there is 

 no visible unconformity in the series, and therefore to the physical 

 geologist they are essentially one group, which suggests many changes 

 in the relations of land and water in adjacent areas, but shows that 

 no new axis of upheaval was developed in the British region. 



The Arenig rocks rest upon the Tremadoc. The deposit contains 

 96 species. Forty Graptolites here make their appearance for the 

 first time ; and of the 3 1 Crustacea only 6 live on from the Tremadoc, 

 and but 3 pass up to the Llandeilo. Of the 149 species in the bed 

 all but 38 are peculiar to it. 



The Llandeilo beds contain 80 genera and 175 species; and 38 

 genera and 73 species pass up from them to the Bala beds. The 

 fauna of the Bala group admits of subdivision into three. The 

 Middle Bala contains 610 species, of which only 102 pass up to the 

 Lower Llandovery. The Lower Llandovery beds, though easily dis- 

 tinguished from the underlying beds by their sandy character, show 

 no unconformity in Central Wales, but near Llandovery, Sir Andrew 

 Ramsay and Mr. Aveline recognise a slight unconformity at Noeth- 

 Grug. These beds contain 204 species, of which 104 pass up. 



Between the Lower and Upper Llandovery there is every- 

 where a perfect unconformity ; and Sir A. Ramsay remarks that the 

 newer beds rest on the denuded edges of the Lower Llandovery, 

 sometimes on the Caradoc sandstone of the Bala series, and at Builth 





