170 E. B'ICKS ON THE SUCCESSION OE 



hitherto been described in my papers to the Society, and some of 

 the facts have been bnt recently made out*. 



Aeenio and Llandeilo Groups. 



History of the Names. — In a paper read by Professor Sedgwick 

 before the Geological Society on the 25th Feb., 1852, he describes 

 the "Arenig slates and porphyries" as forming a distinct and well- 

 marked subgroup in his previously named " Ffestiniog group/' and 

 as resting conformably on the underlying group, to which he had 

 given the name of " Tremadoc slates." This is the first description of 

 the series under the above heading that I have been able to find ; 

 but in papers read by him before the Society on Nov. 29th, 1843, 

 and on Dec. 16th, 1846, the series is frequently referred to, and de- 

 scribed as "a great group of roofing-slate, and contemporaneous 

 porphyry," the former passing into grits, flags, &c, and comprised in 

 his Snowdonian group. The series is also stated to occur in the 

 chains of Arenig, Aran Mowddwy, and Cader Idris, and in the 

 Ffestiniog mountains ; and sections are given in which its position 

 is clearly marked. In his ' Synopsis of the Classification of the 

 British Palaeozoic Rocks,' published in 1852-55, he adopts the 

 same arrangement as in his paper in 1852. Since that time but 

 little has been done to elucidate the position of the group in North 

 Wales, and the name has almost been allowed to lapse. Mr. 

 Salter's researches in portions of the series, however, have been 

 of very great importance, and will be fully referred to when these 

 parts are described. 



The Llandeilo group was named by Sir E. Murchison about the 

 year 1834, and was intended by him to indicate a series of calca- 

 reous flagstones, schists, and sandstones exposed in the neighbour- 

 hood of Llandeilo in Carmarthenshire, and about Builth in Bre- 

 conshire. The exact position of the group in the succession, how- 

 ever, was a doubtful point for many years afterwards, and it was 

 not made at all clear until the districts were examined by the Geolo- 

 gical Survey in 1842, and then chiefly through the labours of Prof. 

 Bamsay. The description given of it at this time (1842), as deve- 

 loped at Llandeilo and Builth, was a series of schists, flags, and lime- 

 stones, with interstratified trap at the base, the whole underlying 

 conformably the Caradoc or Bala group. As here defined, it does not 

 include properly any beds of the age of the great series previously 

 discovered by Prof. Sedgwick in North Wales, and afterwards de- 

 scribed by him in the above-mentioned papers as " Arenig slates and 



* Memoirs published on the rocks of St. David's in addition to those men- 

 tioned in the Quart. Journ. Geol. Soc. Nov. 1871 : — 



Hicks, Trans. Liverpool Geol. Soc. 1869. 



Harkness and Hicks, Quart. Journ. Geol. Soc. Nov. 1871. 



Hicks, Geol. Mag. Dec. 1869 ; Quart. Journ. Geol. Soc. May 1872 ; Trans. 

 "Brit. Assoc. 1872. 



Hopkinson, Trans. Brit. Assoc. 1872. 



Hicks, Proc. Geologists' Assoc, vol. iii. 1873; Quart. Journ. Geol. Soc. Feb. 

 1873 ; Trans. Brit. Assoc. 1873. 



