12 



TKE GEOLOaiST. 



Professor Sedgwick, in the same paper, recognizes the Plymouth 

 group in the slates of Looe, Polperro, and Fowey, in Cornwall.* 



Accepting, at least provisionally, this chronology, we have, when 

 considered chronologically as well as geographically, what, as a mat- 

 ter of convenience, may be called five fossiliferous areas ; namely, a 

 deposit of the age of the Plymouth group in each of the districts. 

 South Devon, North Devon, and Cornwall ; and one of the Barn- 

 staple age in each of the two latter. To avoid repetition, they will 

 be spoken of throughout this paper as Lower South Devon, Lower 

 North Devon, Lower Cornwall, Upper North Devon, and Upper 

 Cornwall. The terms "Upper" and "Lower" are to be understood 

 as applied relatively to the rocks of Devon and Cornwall only, and 

 not as embodying or implying any opinion respecting the co-ordina- 

 tion of these rocks with deposits of the Devonian age elsewhere. 



Had existing materials warranted, it would have been desirable to 

 have made a further division, namely, one having reference to the 

 mineral character of the deposits, as well as to time and place; for it 

 is certain, as might have been expected, that in the same area some 

 fossils are peculiar to the argillaceous beds, and others are found 

 only in the calcareous strata; thus, for example, I learn from Mr. 

 Godwin- Austen that he has found the remarkable coral Pleuro- 

 dictyum prohlematicum in the slates, but not in the limestones, at 

 Ogwell, in South Devon. My own experience is in harmony with 

 this. I have found specimens of the same fossil in the slates at Tor- 

 quay, and hundreds of them occur in rocks of the same character at 

 Looe, in Cornwall, but not a trace of it in limestone anywhere. 

 The two species of sponges belonging to the genus SteganocUctuum 

 of Professor M'Coy occur in the slates along the entire coast of 

 Cornwall, from Powe}'- Harbour to the Kame Head ; at Bedruthen 

 Steps in the north of the same county ; and at Mudstone Bay, near 

 Brixliam, in South Devon ; but have never been met with in cal- 

 careous strata. At present, however, it would be premature to at- 

 tempt a division of this kind. 



My present object is to give some account of the amount and 

 character of tlie Devonian population of the five areas as above 

 defined, when the census was last taken. The inquiry as to cha- 

 racter goes no further than to ascertain to what extent they were a 

 migratory or colonizing race. 



Having spent a considerable portion of the leisure I have been 

 able to command during the last twenty years in collecting and 

 studying the fossils of the districts under consideration, especially 

 along the entire line of coast extending from Polperro in Cornwall 

 to Torbay in Devonshire, and also at South Petherwin, I have natu- 

 rally been led to pay some attention to their distribution in time 

 and space ; and several concurring circumstances have recently 

 brouglit the subjects more prominently before me. Amongst other 

 tilings I may mention a passage in the recent address of Professor 



* QviiU-lorh Journal Geol. Soc. vol. viii. p. li. 



