ANNIVEESAEY ADDEESS OF THE PEESIDENT. 



Ill 



the Survey not recognizing the two divisions termed Lower and Upper 

 on the maps, as they are not separable or distinguishable in the field 

 over large areas. The exact position of the Arenig group was, and 

 has for many years been, a doubtful question ; and it is due to the 

 continued researches of Messrs. Salter and Homfray in North Wales, 

 and of Dr. Hicks in South Wales, that the name and position assigned 

 to these beds by Prof. Sedgwick in 1843 is now revived and defi- 

 nitely established, and that the Arenig group of rocks and fossils 

 has now had assigned to it its true stratigraphical place above the 

 Tremadoc group and below the Llandeilo proper, and with a fauna 

 recognized as peculiarly its own. The Llandeilo of Murchison and 

 the Survey (first named in 1843) did not really include any of the 

 Arenig proper as determined in North Wales in 1843, 1846, and 

 1852, or as occurring in the chain of the Arrans, Arenig, Cader Idris, 

 and in the Ffestiniog region (typical districts). The term " Lower 

 Llandeilo " of Murchison included Prof. Sedgwick's " Arenig," not 

 as first intended ; and Prof. Ramsay, in the first edition of his 1 Geo- 

 logy of North Wales states that " since 1848 the Survey con- 

 sidered the slates close below and above the Arans and Arenigs as 

 equivalent to the Llandeilo flags of Builth and Shelve " f . 



Mr. Salter, however, in the appendix to the same work in 1866, 

 pp. 253-257, under the section " Lower I^landeilo " and plates 8-12, 

 clearly showed the importance of truly correlating the Arenig group, 

 eliminating it partly from the Upper Tremadoc below and from the 

 so-called Llandeilo above. 



There is now no doubt whatever about the horizon or position 

 of the Arenig rocks. They are entirely distinct from the Tremadoc 

 group of Sedgwick, and were claimed as Lower Llandeilo by 

 Murchison. They are the " Arenig and Skiddaw " group of Sedg- 

 wick, established by him in 1834, and confirmed through subse- 

 quent research in the Skiddaw area, and of which the fossils were 

 described by M'Coy before 1851. Priority therefore under all heads 

 is due to the researches of Prof. Sedgwick. Finally Salter described 

 the majority of the fossils of both the Arenig and Tremadoc groups, 

 showing in the clearest manner that the Tremadoc rocks were the 

 natural termination of the Ffestiniog or " Middle Cambrian " series, 

 and the Arenig group the base of the " Upper Cambrian " or 

 " Lower Silurian " of Murchison ; these rocks through their fossils, 

 have their equivalents, as shown by Salter, in the Quebec group of 

 Canada. 



Plants. — None. 



Peotozoa. — None. 



Hydeqzoa. — Subsequent to the change of conditions which ter- 

 minated the Tremadoc period, or at the coming-in or commencement 

 of the Arenig deposits, no less than 18 genera and 42 species of 



* Mem. Geol. Surv. of Great Brit. vol. iii. p. 6 (ed. 1, 1866). 



t Sir R. Murchison, in 1834, termed the Llandeilo flags " Trilobite schists " 

 in his description of the Shelve country, the Carneddau (Builth), and the 

 neighbourhood of Llandeilo ; and imder this name were included (near Shelve) 

 the strata as low as the base of the Stiperstones, the whole of these rocks being 

 older than the Caracloc sandstone. 



