171 



porphyries ;" nor could it have been intended at the time that it 

 should comprise more than was really to be found in the typical 

 district. . Unfortunately, however, from a mistaken idea of the 

 relative age of the two series, the name Llandeilo was gradually 

 made to include the whole of Prof. Sedgwick's group ; and therefore 

 a meaning was given to the name far greater than could have been 

 at first intended, or than it had the slightest claim to — as may be 

 seen by referring to ' Siluria/ 2nd edition, 1859, and to Prof. 

 Ramsay's ' Memoir on North Wales,' 1866, where it is mentioned 

 that " since 1848 the Geological Survey have been in the habit 

 of considering the slates close below and above the Arans and 

 Arenigs &c. of North "Wales as equivalent to the Llandeilo flags 

 of Builth and Shelve." The group, as it occurs in the typical 

 Llandeilo district, is sufficiently important to occupy a good position 

 in the classification; and more should not be asked for it. The 

 relative position of the two groups is that of two distinct but con- 

 formable series, intervening between the Tremadoc group on the 

 one hand and the Bala group on the other ; and the proportion of 

 the series at St. David's now given to each group is that -which, on 

 comparison with the sections in the typical districts, each seems to 

 have a real claim to. If, therefore, the arrangement should appear 

 to differ in some of its details from that previously adopted in 

 describing these rocks, I must ask for it the recognition due to 

 more recent observations, to a careful comparison with sections in 

 most other Welsh areas, and to a desire to adopt the lines only for 

 the subdivisions which nature seems to indicate by combined pakeon- 

 tological and lithological evidence. 



Aeekcg Group. 



The rocks in the neighbourhood of St. David's, which I propose 

 to group together under Prof. Sedgwick's name, " Arenig," consist 

 for the most part of black slates. They lie conformably on the 

 Tremadoc group along the north side of the anticlinal, and they 

 may be traced in an unbroken course for from six to seven miles, 

 the strike of the beds varying only as they curve with the general 

 axis. They also occur at Ramsey Island, faulted against beds of 

 the Harlech group, having been brought down along with some 

 beds of the Tremadoc group and Lingula-flags. The chief fault 

 which caused this change in their position is the one indicated by 

 the line of Ramsey Sound ; but it was doubtless assisted by the 

 others now visible in the island. The rocks indicate the pre- 

 valence of a deep sea when they were deposited, and show that the 

 depression of the sea-bottom, which had been gradually taking 

 taking place during the deposition of previous groups, had now 

 become much more decided. The depression also seems to have 

 culminated early in the Arenig period, and then to have continued 

 much in that state during the remainder of the epoch. The group 

 contains altogether near 4000 feet of these deep-sea deposits ; and, 

 considering the very slow rate at which such deposits must have 



