CHAPTEE in. 



ORDOVICIAN PERIOD. 



1. Stratigraphical Evidence. 



TjlNGLAND. The most southern area of Ordovician 

 -*-^ rock in England is a small tract on either side of 

 Dodman Head on the east coast of Cornwall. The rocks 

 exposed consist of brown grits, quartzites, slates, and ash- 

 beds, with some conglomerates, and they contain fossils 

 of Bala age, but as their base is not visible it is impossible 

 to draw any certain inferences as to the neighbourhood of 

 land. It is true that Mr. G-od win -Austen has called the 

 conglomerates " shingle-beds," ] and infers that they were 

 formed in close proximity to a coast-line, but Professor 

 Sedgwick describes them as trappean and schistose con- 

 glomerates, and speaks of their passing into " schaalstein" 

 and " trap shale " (i.e. volcanic ash), so that they hardly 

 seem to warrant the inference drawn by Mr. Austen. 



At Malvern there is a total absence of Ordovician strata, 

 the Upper Cambrian shales being unconformably covered 

 by Silurian, but as the former dip under the latter, it is 

 possible that Ordovician rocks exist beneath the Silurian 

 area to the westward, and it would not, therefore, be safe 

 to infer that the Malvern area was land throughout Ordo- 

 vician times, though it will be seen from the sequel that 

 land did then exist over the centre of England, and it is 



1 " Quart. Journ. Geol. Soc.," vol. xii. p. 44. 



