72 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Oolite series is not likely to offer any serious impediment in the way 

 of researches for coal, nor of enquiries as to the structure and arrange- 

 ment of the rock-series underlying the district bordering on the 

 Thames Valley. 



§ 5. Nummulitic group. 



The axis of Artois has hitherto been viewed (D'Archiac, I. c.) 

 with reference to its influence on the characters of the Cretaceous 

 and Nummulitic series, as these are presented both along and on 

 either side of it ; the influence being just such as would be exercised 

 by a great physical limit, and which is indicated both by the compo- 

 sition of the beds as they overlap, and by the included marine fauna : 

 but this influence is far more strongly marked in the Cretaceous 

 series than it is in the Tertiary. 



As a general rule, the members of the Nummulitic group thicken 

 away on either side of this axis (to the north and south), thus show- 

 ing that removal has taken place along the higher portions of the 

 line, over every part of which it originally passed. The Chalk strata 

 also thicken away in the same direction ; so that, although from the 

 Boulonnais to the province of Hainault the Nummulitic strata are 

 found resting on chalk only, it is on the lower parts of that series 

 and on reduced portions of it that it is seen along the central ridge, — 

 indicating that precisely the same movements, attended by the same 

 results, were repeated along the very same line at two very distinct 

 periods of time ; — 1st, subsequently to the completion of the Creta- 

 ceous series ; 2ndly, after the accumulation of the Lower Nummu- 

 litic group. 



Chalk strata have been largely removed from off the surface of 

 the province of Hainault, and this clearing must have taken place 

 before the Nummulitic period, as beds of that age there overlie the 

 truncated edges of the Chalk, and cover the old Palaeozoic series. In 

 the Boulonnais, to the W. of the axis of Artois, although Nummu- 

 litic beds come close to the edge of the chalk-escarpment, they are 

 not found loithin that area. Here too the denudation has extended 

 to the whole thickness of the Cretaceous series. This, therefore, 

 has been effected subsequently to the Nummulitic period ; although 

 the two periods are so wide apart in time, the amount of removal 

 over the Boulonnais during some post-Nummulitic period corresponds 

 exactly with what was effected at the Belgic extremity of the axis, 

 during the interval between the Cretaceous and Nummulitic groups. 



There are many points on which I have not thought it proper to 

 inquire in the preceding pages ; of these perhaps the most important 

 is that of the arrangements of the Palaeozoic rocks from Therouanne 

 to Calais. We have, however, evidence to warrant the following 

 general inferences : — 



1st. That the physical configuration of Western Europe at the 

 close of the Palseozoic period is sufficiently clearly defined to admit 



