228 MR. J. W. D. ROBINSON ON [vol. lxxvi, 



•6. The Devonian of Ferques (Lower Boulonnais). By 

 John William Dudley Robinson, M.Sc, F.G-.S. (Read 

 April 21st, 1920.) 



[Plate XIV — Map and Section.] 



Introduction. 



The Palaeozoic rocks of the Boulonnais are classic ground, and 

 received the attention, among others, of De Verneuil, Murchison, 

 Rigaux, and above all Godwin- Austen and Gosselet. Owing to the 

 occurrence of coal, however, most attention has been given to the 

 Carboniferous deposits, and, except for paheontological work by 

 Rigaux, the Devonian seems to have been comparatively neglected 

 for the last forty years. The classic description is that published 

 by Godwin-Austen in 18-53, which has been slightly amended by 

 Gosselet and Rigaux, while the beds have been correlated with 

 those of other areas by Gosselet. 



The published maps are poor, and of little use in the field. This 

 is partly due to the lack of a good topographical map, but also to 

 the apparent absence of detailed mapping. The accompanying map 

 (PI. XIV) suffers from the first defect, but was prepared on a scale 

 of 1 : 1000 over an area extending across' the outcrop along the 

 Nord Railway, and on a scale of 1 : 10,000 for the remainder. 



All the beds are referred to the Upper and to part of the Middle 

 Devonian (Famennian, Frasnian, and Givetian). They consist of 

 limestones, sandstones, shales, dolomites, and conglomerate. 



Physical Aspect. 



The Lower Boulonnais maybe regarded as forming the southern- 

 most extension of the Weald. It is ringed round b} r Chalk hills ; 

 but the denudation in the centre has been more extensive, exposing 

 a considerable expanse of Jurassic rocks and a smaller one of the 

 Palaeozoic. The district is drained by a number of small streams, 

 the courses of which were probably determined when the Cretaceous 

 deposits extended over the whole area. 



The country rises generally in a slight swell, from the base of 

 the Chalk hills to the Devonian plateau ; but the differences in level 

 are slight, and the valleys shallow. 



The Devonian rocks disappear under the Cretaceous on the north 

 and east, under the Jurassic on the west, and are bounded by the 

 Carboniferous on the south. 



The soil is poor ; some portions appear to have been recently 

 brought under cultivation, but much is still covered by woods, 

 rough commons with gorse and broom, and bogs. 



A line of quarries stretches across the outcrop from end to end, 



