ANNIVERSARY ADDRESS OF THE PRESIDENT. XXXIX 



3rd. A certain area of sea-bottom had no strata deposited upon it 

 for a long period of time. 



In the first case, so large a denudation implies unconformity and a 

 great interval between the last Tremadoc bed and the first of Llandeilo 

 age, and we have thus a coincidence of lapse of time and change of 

 genera and species. 



In the second case, if there were in places direct succession between 

 Lingula and Tremadoc beds, why, on a given horizon, does the 

 Lingula-flag fauna disappear ? And the third supposition also by no 

 means helps us out of the difficulty of a complete change of life in 

 those areas where the three formations occur in direct order of 

 superposition. 



The question thus arises, why are they so perfectly distinct ? 

 Though in the present state of our science we cannot clearly solve 

 this question, I hope, ere closing this Address, to lend a little aid 

 towards its future solution. 



One point, if true, is important, and bears strongly on this ques- 

 tion. I believe that the Llandeilo flags must lie as unconformably 

 on the underlying strata in Wales as they do in Scotland ; and a 

 close analysis of the structure of the country strengthens this belief. 

 Thus, in Merionethshire the Lingula-flags are from 5000 to 6000 feet 

 thick ; in Caernarvonshire, near Llanberris, they are only about 

 2000 feet thick, having, in a space of about 11 miles, lost 4000 feet in 

 thickness, either by thinning of the whole mass or by overlap. The 

 latter seems probable; for in Anglesey, and perhaps even on the 

 Menai Straits, the Llandeilo and Bala beds lie directly on Cambrian 

 strata, both Lingula- and Tremadoc Slates being absent. The same 

 is the case in Ireland. 



But the Caernarvonshire and Anglesey areas are so near that 

 this sudden disappearance of all the Lingula-flags in a few miles 

 proves an overlap so rapid that I can scarcely doubt that it implies 

 an actual unconformity of upper on lower strata ; and the lapse of 

 stratigraphically unrepresented time thus indicated is associated, 

 in my mind, with a total change of fauna between the Tremadoc 

 formation and the overlying Llandeilo flags ; or, in other words, the 

 period of which we have no fossils preserved, represented by the 

 unconformity, was so long that all the old life had become, for some 

 reason, thoroughly remodelled before the deposition of the Llandeilo 

 flags began. 



The same kind of reasoning applies to the difference of species in 

 the Lingula-flag and Tremadoc beds. 



Llandovery or Pentamerus-beds. — The evidence of the physical 

 and palseontological relations of the Llandovery beds to the under- 

 lying strata is in part more direct. 



In North Wales, in Montgomeryshire and Merionethshire, the 

 Lower Llandovery beds, being sandy, are easily separable from the 

 slaty Bala beds beneath, and there is no very direct evidence of un- 

 conformity between them; but in South Wales, near Llandovery*, 

 * Observed and mapped by Mr. Areline. 



