634 Professor Sedgwick and R. I. Murchison, Esq., on the 



order of the successive formations north of Dartmoor, but also, as far as we 

 are able, to bring them into comparison with the formations which are ex- 

 panded from the south side of the Dartmoor granite to Start Point, and to 

 the other headlands of the south coast of Devon. We believe that our pre- 

 sent attempt is almost entirely new; for though some portions of the county 

 have been described in considerable detail, yet the several parts have not been 

 brought into any distinct comparison with one another ; and in all the pub- 

 lished geological maps, the greater part of the slate-rocks have been repre- 

 sented under one colour, without an attempt either to separate them into 

 distinct formations or to mark their succession*. Still less has any attempt 

 been made to bring them into a strict geological comparison with the other 

 mountain groups of England and Wales. Our present object is to exhibit a 

 synopsis of the evidence by which the successive formations are determined. 

 But in future and longer communications, we hope to lay more ample details 

 before the Society, and to describe a large number of organic remains, by 

 help of which, the successive deposits may be referred still more precisely to 

 their true place in the series of British formations, inferior to the carboniferous 

 groups. We are aware of the great difficulty of our task, and if we can suc- 

 ceed in taking only the first step towards its accomplishment, our labours will 

 be well rewarded f. 



Chap. I. — Five Geographical Regions of Devonshire. 

 (See Map, PI. L.) 

 Considered geologically, and under the most general point of view, Devon- 

 shire may be divided into at least five distinct geographical regions. 



* Mr. Greenough represents the slate-rocks under two colours. But the separation is founded 

 on mineral characters and not on age ; for even a portion of the culm series, altered near the gra- 

 nite, is represented of the same colour with the oldest slate-rocks of Cornwall. 



Rev. J. J. Conybeare made also a separation of the Cornish and Devonian slates into two 

 groups, partly on the same principle, and certainly with no reference to any ascertained order of 

 superposition. His papers, which he did not live to finish, contain some excellent details. Mr. 

 H. T. De la Beche, in the first edition of the Ordnance Map of Devon, (which had been published 

 sometime before our examination of the county,) included (with the exception of the district of 

 Start Point) the whole slate series and the culm-measures under one colour, not attempting any 

 separation of them. 



t The details alluded to in the Introduction are now given in Part II. of this paper. The whole 

 text of Part I. is printed (with the exception of verbal corrections and an expansion of one or 

 two mere mineral descriptions,) from the communication made by the authors in June 1837. A 

 few sentences at the end of it, in which the oldest rocks of North and South Devon were placed 

 provisionally in the Upper Cambrian and Lower Silurian system, are now omitted, as the authors, 

 after a more mature examination of the fossils, are convinced that, so far, their first conclusions 

 were erroneous. This omission will be alluded to again in the Introduction to Part II. (June 1839.) 



