78 Physical 



vonshire there is, as yet, no 'certainty of the existence 

 of the Tremadoc slates, and these ought to be found 

 overlying the Lingula flags if the whole of the Silurian 

 series were present. Still further, as we approach 

 Caernarvon and Bangor, even the Lingula flags are 

 absent, and the Arenig slates are found lying directly 

 on the purple slates and conglomerates of the Cam- 

 brian series all the way from Bangor to Caernarvon. 

 This clearly shows that the base of the Arenig slates 

 has overlapped the whole of the Tremadoc slates and 

 Lingula flags, in the area between the Ffestiniog and 

 Portmadoc country and the neighbourhood of the 

 Menai Straits, and an overlap so great means uncon- 

 formity between the strata ; or, in other words, in this 

 area the strata of older date than the Arenig slates had 

 been raised above the sea, and subjected to sub-aerial 

 agencies of denudation, while the deposition of the 

 Arenig slates was going on elsewhere. In this manner, 

 therefore, it happens that the Arenig slates are now 

 found resting directly on the Cambrian strata, without 

 the intervention of the missing members of the series, 

 viz., the Tremadoc slates and Lingula flags ; and still 

 further north, in Anglesea, these strata are also wanting. 



The effect of this episode in the physical geography 

 of the area seems to have been, that at this period a 

 tract of land lay in the north- west of what is now Wales, 

 and probably far beyond that district during the de- 

 position of the Arenig strata on its borders, but what 

 the features of that land were I cannot say, except that 

 it may have extended to Ireland, where there is a 

 similar unconformity, the Lingula flags and Tremadoc 

 slates being also wanting in Wicklow. Probably the 

 whole region was low and unimposing. 



The next question that arises is, what was the 



