TRANSACTIONS OP SECTION C. 195 



in fossils have been found either in the Combe Martin or Lower Ilfracombe 

 Beds, and they may be either Upper-Middle Devonian (Givetian) or Lower- 

 Upper Devonian (Frasnian). Unfortunately no goniatites have been found in 

 the Devonian of North Devon, so that exact con-elation is difficult. In higher 

 portions of the Ilfracombe Beds Spirifer verneuili and Rhynchondla (Wilsonia) 

 cuboides are found, which are sufficient to establish the Upper Devonian (pre- 

 sumably Frasnian) age of the rocks. The highest Ilfracombe Beds are less 

 calcai'eous, and there seems no reason to doubt that they pass upwards con- 

 formably with the Merte iSlates, the Upper Devonian age of which is com- 

 pletely established by the occurrence of Spirifer veriieuiU (var. hamlingi), and 

 they may well represent the Schistes de Matagne, which form the highest beds 

 of the Frasnian in the Ardennes. Rocks of the same age appear to be met with 

 in the Boulonnais and in the boring in Tottenham Court Road in London. The 

 Morte Slates become more arenaceous at the summit, and are probably suc- 

 ceeded conformably by the Pickwell Down Sandstones. The junction is usually 

 faulted, but this is apt to be the case in strongly folded areas where succes- 

 sive beds differ considerably in physical characters, and the resistance they 

 offer to the forces to which the rocks have been subjected. The Pickwell Down 

 Sandstones have yielded the typical Upper Old Red Sandstone fish Holopty- 

 rhius and Bothriolepis, and may be compared to the beds with the same forms 

 reached by a boring at Southall, west of London, and to the Psammites de 

 Condroz. They must be referred to the terrestrial or Old Red Sandstone type 

 of the Famennian. The Baggy and Marwood Beds that overlie the Pickwell 

 Down Sandstone and the lower portion of, the succeeding Pilton Beds represent 

 a marine facies of the Upper Famennian, as well as the Calcaire d'Etrceungt, 

 which forms a passage to the Carboniferous in the Ardennes. The Upper 

 Devonian of the Turnford boring in the Lea Valley, north-east of London, and 

 the marine beds of the Upper Old Red Sandstone of South Wales and the 

 Coomhola Grits in South Ireland are probably at about the same horizon as the 

 Baggy and Marwood Beds and the base of the Piltons. The Upper Pilton 

 Beds have now been shown to be of Carboniferous and not Devonian age, and 

 to extend upwards and include the basement beds of the Zaphrentis zone. 



It will be seen that North Devon is characterised by a repeated alternation 

 of the terrestrial or Old Red Sandstone facies formed of materials laid down 

 by rivers or in lakes or transported by the action of the wind and the marine 

 facies of the Devonian. This alternation is even more remarkable here than 

 that in the Eastern Baltic, with which all students of geology are familiar. 

 There were three periods when the marine recession resulted in the deposition 

 of the former, the first commencing in some areas towards the close of Silurian 

 times. Each recurrence of the terrestrial facies is characterised by a com- 

 pletely different fauna and flora. There were also three periods of marine 

 transgression, one about midway in the Lower Devonian, the second in the 

 Givetian and Frasnian, and the third commencing near the close of the Famen- 

 nian and reaching its maximum in the Carboniferous. In South Devon and 

 Cornwall the conditions as a whole were more marine than in North Devon, and 

 it was only during the deposition of the Dartmouth Slates in North Cornwall 

 that entirely terrestrial (in this case fresh-water) conditions prevailed. In 

 South Wales, on the other hand. Continental conditions were more prevalent 

 than in North Devon, but a far more important difference between the north 

 and south of the Bristol Channel lies in the complete omission, due either to 

 non-deposition or erosion, of any representative of North Devon strata from at 

 least low down in the Lynton Beds to the summit of the Morte Slates. 



The variation of conditions of deposition which are so strongly marked in 

 North Devon can be traced in most of the occurrences of Devonian rocks in 

 other parts of the world, though they are nowhere else so striking and unam- 

 biguous. It is the deepening and transgression of the sea in Givetian and 

 Frasn-an times that is tlie most widely extended and most strongly marked of 

 nil these changes. 



The Devonian period is not a natural division of the history of marine sedi- 

 mentation characterised by a gradual deepening and subsequently a gradual 

 shallowing of the ocean waters, but was determined solely by the interval 

 between the last marine beds of the Silurian and the earliest marine beds of 



