Dr. J. W. Evans — Devonian Rocks of North Devon. 549 



and to extend upwards and include the basement beds of the 

 Zaphrentis zone. 



It will be seen that North Devon is characterized by a repeated 

 alternation of the terrestrial or Old lied 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 than that in the Eastern Baltic, 

 witli which all students of geology are familiar. There were, as we 

 have seen in North Devon, three periods when the marine recession 

 resulted in the deposition of the Old lied type of sediment with fresh- 

 water fossils, the first commencing in some areas towards the close of 

 Silurian times and continuing through the Gedinnian, the second 

 including the uppermost horizons of the Lower Devonian and 

 apparently the whole of the Eifelian, and the third in theFamennian. 

 Each recurrenceof theterrestrialfacies is characterized by a completely 

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

 transgression, one, which is missing in the Baltic area, about 

 midway in the Lower Devonian, the second in the Givetian and 

 Erasnian, and the third commencing near the close of theFamennian 

 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 (here freshwater) 

 conditions prevailed. In South "Wales, on the other hand, terrestrial 

 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 unambiguous. It is the deepening and trans- 

 gression of the sea in Givetian and Frasnian times that is the most 

 widely extended and most strongly marked of all these changes. 



The Devonian period is not a natural division of the history of 

 marine sedimentation characterized 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 the Carboniferous Limestone 

 on the "Welsh Border, where these strata were first studied in the 

 early days of stratigraphical research. It was in this way that the 

 limits of the Old Red Sandstone were originally fixed, and as 

 a consequence also those of the contemporaneous rocks of marine 

 origin elsewhere deposited, which were a little later grouped together 

 to form the Devonian. 



