Page:The Geologist, volume 5.djvu/26

12 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 matter 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 Barnstaple 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-ordination 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 Pleurodictyum problematicum 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 Torquay, 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 Steganodictyum of Professor M'Coy occur in the slates along the entire coast of Cornwall, from Fowey Harbour to the Rame Head; at Bedruthen Steps in the north of the same county; and at Mudstone Bay, near Brixham. in South Devon; but have never been met with in calcareous strata. At present, however, it would be premature to attempt a division of this kind.

My present object is to give some account of the amount and character of the Devonian population of the five areas as above defined, when the census was last taken. The inquiry as to character 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 naturally been led to pay some attention to their distribution in time and space; and several concurring circumstances have recently brought the subjects more prominently before me. Amongst other things I may mention a passage in the recent address of Professor