684 Geological Society. 



numerous, but the acid rocks which build one large sill repeated by 

 strike -faulting, have the greater bulk. Progressive deep-seated 

 differentiation of the granophyric magma has given rise to a series 

 of small intrusions near the main sill. The rocks in these minor 

 intrusions show all transitions from eurites, through markheldites, 

 to quartz-diabase, but all agree in showing granophyric structures, 

 and they are usually arranged in an upward order of decreasing 

 density. The main sill is the youngest intrusion in the district, 

 and is of uniform composition throughout, except that differentia- 

 tion in place has resulted in the production of narrow basic selvages 

 at both the upper and the lower margins. Petrographical and 

 structural considerations unite in justifying the assumption of a 

 pre-Upper Bala date for all the intrusions. 



Faulting and folding were alreacT/v in progress before the in- 

 trusion o£ the hypabyssal rocks. The faults and the igneous rocks, 

 both bedded and intrusive, decrease in importance westwards coinci- 

 dently w r ith a change of strike. The early movement acted along 

 the same lines as the later, more powerful ' Caledonian ' movements. 



April 21st.— Mr. E. D. Oldham, F.R.S., President, 

 in the Chair. 



The following communication w r as read : — 



' The Devonian of Ferques (Lower Boulonnais).' Bv John 

 William Dudley Robinson, B.Sc, F.G.S. 



In the Lower Boulonnais. between Calais and Boulogne, lies a 

 small tract of Devonian rocks. They form a link between the 

 Devonian beds in Belgium, France, and Germany, and those of 

 England geographically, and also geologically, since they appear 

 to have been formed in a narrow strait which joined the open seas 

 extending towards the Atlantic and over Germany and Russia. 



They have been regarded as undisturbed beds lying on a Silurian 

 land-surface, covered conformably by Carboniferous Limestone, 

 but separated from it by a gap due to cessation of deposition. 

 Detailed mapping shows that the Carboniferous and Devonian are 

 separated by a thrust-fault and that the Devonian has been riven 

 into blocks by dip-faults or ' tears ' caused by the stresses set up 

 by the thrust-faulting. These blocks are seen to have been pushed 

 farther and farther to the north as one proceeds westwards. The 

 failure to recognize this structure has caused beds to be inserted 

 in the succession which are, in fact, merely parts of other bands. 

 The palaeontology, therefore, must be reworked for correct zoning. 



The structure thus shown indicates that the Devonian coast-line 

 may well have taken a northward trend under the Straits of Dover 

 and passed north of the Kent Coalfield and the Thames. 



The more complicated structure of the Devonian thus revealed 

 indicates a corresponding increase of complexity in the Carboni- 

 ferous strata, which may be of importance in unravelling the 

 structure of the Kent and Boulonnais Coalfields. 



The Devonian beds of the area described are practically unaltered 

 and fossiliferous in many parts. Some of the types of rock 

 described have previously escaped notice. 



