Mr. Lyeirs Address. 389 



contain thick beds of limestone, entirely dissimilar in structure and 

 fossil contents from any limestones of the underlying " grauwacke," 

 in which they had previously been merged. The culm measures 

 consist of grit, sandstone, shale and limestone; and these rocks, it is 

 said, are never affected by a slaty cleavage like the lower Silurian 

 and Cambrian rocks on which they rest. From this character, as 

 well as from their prevailing mineralogical structure and imbedded 

 fossil plants, the authors regard the culmiferous formation of Devon 

 as perfectly identical in age with other coal-fields, and as more par- 

 ticularly analogous to the culm-bearing strata of Pembrokeshire ; 

 a part of which also once passed for "grauwacke," but Mr. Mur- 

 chison has recently shown that it belongs to the South Welsh coal- 

 field, which is known by all geologists to rest upon mountain lime- 

 stone. 



Thus referred to the age of our ordinary coa(l, these strata of North 

 Devon are further proved to lie in a great trough, their southern edges 

 being turned up against the granite of Dartmoor, where they acquire, in 

 contact with the granite, when traversed by elvan dykes, many 

 characters of the metamorphic rocks, or those commonly termed 

 primary. The phaenomena of interference and alteration at the 

 junction are such as to give a comparatively modern date for the 

 eruption of the Dartmoor granite, and to explain why so much dif- 

 ficulty and ambiguity has prevailed in determining the age of some 

 of the altered culm beds. 



Among other points which this survey of Professor Sedgwick and 

 Mr. Murchison has settled, so far as Devon is concerned, is one of 

 the highest theoretical interest, and on which for more than two 

 years the Society has been anxiously desiring more accurate infor- 

 mation; 1 allude to the true stratigraphical position of certain 

 shales near Bideford in North Devon, containing fossil plants of the 

 same species as those which are found abundantly in the coal. I may 

 first remind you that a discussion had previously arisen respecting 

 the alleged discovery by Mr. Weaver of anthracite, with the usual 

 carboniferous plants, in the graywacke or transition rocks of Ire- 

 land*. Notwithstanding the value justly attached to the opinion of 

 so experienced and long-practised an observer, your Council hesi- 

 tated to print his statement, and requested him to reexamine the 

 ground. At the same time Mr. Griffith, to whom we are looking 

 for the publication of a Geological Map of Ireland, had come to a 

 different conclusion, and Mr. Weaver having been induced to repeat 

 his observations, became convinced that he was in error, and has 

 since studiously availed himself of every opportunity of announcing 

 this change in his views. 



You are aware that as yet in the British islands, scarcely any vege- 

 table impressions have been met with in rocks more ancient than the 

 carboniferous strata above the old red sandstone, so that we know not 

 what species of plants belong to the graywacke or transition group. We 

 can only presume from analogy that since the shells, corals, and other 



* Phil. Mag. and Annals, vol. viii. p. 147. 



