206 J. B. Hill— The Palceozoics of West Cornwall. 



ni. — On the Relation between the Older and Newer Pal^ozoios 



OF West Cornwall.' 



By J. B. Hill, R.N., of the Geological Survey. 



(PLATE XIV.) 



Introductory. 



f pHE author, who has long been engaged on the Palfeozoics of 

 \_ West Cornwall, divided the killas extending westward from 

 Oerrans Bay into four groups that formed a natural sequence.^ 

 Moreover, as they included definite Lower Silurian horizons, as 

 characterized by the fossiliferous quartzite of Carne, these divisions 

 were linked with the Lower PalEeozoics. They consist of the 

 Verj^an, Portscatho, Falmouth, and Mylor groups. On the latest 

 issues of the old Survey maps the area occupied by the first of these 

 divisions is coloured as Silurian, and the region occupied by the 

 remainder as Devonian. That colouring, however, was not adopted 

 by De la Beche, who surveyed the region, nor was it the result of 

 any subsequent survey of the ai-ea. In the original Geological 

 Survey map of Cornwall the killas was separated by De la Beche 

 into two divisions, viz., a grauwacke group and a carbonaceous 

 series. Thus, the former lying below the Culm -measures was 

 undifferentiated for the reason, as explained in his Report,^ that the 

 progress of geology at that time only warranted the broadest 

 generalizations. He moreover expressed the opinion that the 

 terms Cambrian and Silurian should be restricted to the areas that 

 gave rise to the prolonged researches of Murchison and Sedgwick, 

 and deprecated the extension of that nomenclature to districts that 

 had not received the same detailed investigations. In a later and 

 undated issue of the map the grauwacke group is divided into 

 Devonian and Silurian, presumably by the authority Sir Roderick 

 Murchison. The Devonian colour was not only applied to fossili- 

 ferous strata in East and Mid Cornwall, but was extended over the 

 uufossiliferous strata in the west. The Silurian tint, on the other 

 hand, was restricted to a zone that had yielded organic remains. 

 Murchison, however, was of opinion that the older zone extended 

 far beyond those limits into the barren strata coloured as Devonian,* 

 and it is evident that the latter tint was adopted as a matter of 

 convenience, as no re-examination of the area seems to have been 

 undertaken. The known Silurian region was confined to the 

 coastal belt between Chapel Point and Gerrans Bay, a boundary 

 connecting those localities admitting of the ready isolation of that 

 zone from the rest of the country. That such a broad generalization, 

 however, was only regarded as provisional may be inferred from the 

 absence of a line on tlie map between the two divisions. It will be 

 seen, therefore, that the subdivision of the killas, as the result of the 



' Communicated by permission of the Director of the Geological Survey. 

 - Summary of Progress, 1898, p. 97, and Trans. Eoy. Geol. Soc. Cornwall, 

 vol. xii (1901), pt. 6. 



* " Report on the Geology of Devon, Cornwall, and "West Somerset," pp. 38-41. 



* Trans. Eoy. Geol. Soc. Cornwall, vol. vi, p. 322. 



