AUSTEN — EXTENSION OF THE COAL-MEASURES. 65 



Preux, S.E. of Arras. This intermediate point gives an approxi- 

 mate outline for the range of the OoUte series beneath the Chalk of 

 the North of France ; and this, it will be seen, is parallel with the pre- 

 sent axis of Artois : we hence infer that the limitation of the Oolitic 

 group arose from the same physical barrier along the whole line. 



Taking the two extremities of this line, there is this difference to 

 be observed, that whereas on the E. the series commences with the 

 lower and extends to the middle Oolites, on the W. it commences 

 with the middle and is continued to the upper. 



In the Boulonnais the Oolitic group is brought up against the old 

 ridge here in question by a process of overlap, a circumstance which 

 indicates contemporary subsidence. On the E. the upper Oolitic 

 beds have been removed as they approach the ridge, and the surface 

 thus produced is overlaid by the lowest beds of the Cretaceous series : 

 or, confining ourselves for the present to the Belgic-French area, it 

 is clear that the destruction of the Oolitic surface commenced on the 

 E. before it did on the W. ; thus showing a movement correspond- 

 ing in order and results with what took place at a subsequent date 

 with respect to the Cretaceous beds. 



The original limitation of the Oolitic group, and the subsequent 

 abrasion of some of its beds, were both dependent on a rise of old 

 Palaeozoic strata, of which the trend, for one part at least, was in 

 conformity with the line of Artois ; but this line, as we have seen, 

 was not then the mere ridge that it is at present, — it became an axis 

 by the subsidence of the tract which extended to the N., and this 

 took place gradually, during the course of more than one subsequent 

 geological period. 



The line of the chalk axis of Artois can be prolonged westwards, 

 by means of like phsenomena of elevation and fracture, into the di- 

 strict of Bath and Frome : whether this line was throughout de- 

 pendent on the same early physical features, is a question which must 

 long continue matter of speculation : what is known is this, that the 

 relations of the Oolitic beds to the Mendip anticlinal are just such 

 as have been noticed in the Boulonnais ; and further, that the 

 Mendip anticlinal emerges from beneath, and is in continuation of, 

 the great axis of Secondary strata of the South of England. 



The littoral limits of the Oolitic series on the N. were dependent 

 on the extent of that area of land-surface which, including portions of 

 Belgium, Holland, and the southern extremity of the German Ocean, 

 became narrower in its extension towards our own area ; so that, if 

 we trace an imaginary line which shall fall in with the anticlinal at 

 Frome, it may possibly represent and complete the tract in ques- 

 tion ; inasmuch as we know that such line represents a zoological 

 boundary — that on either side of which the Oolitic fauna puts on a 

 change: N. of this line the facies is Germanic, to the S. it is Nor- 

 manic*. 



These considerations suggest the probability that the Oolitic series 

 of depositions may be wanting over an area part of which is now 



* Morris, Quart. Journ. Geol. Soc. vol. ix. p. 317. 



VOL. XII. — PART I. F 



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