GEEENSAND AKD PALAEOZOIC EOCKS TJNDEE LONDON. 907 



shales and sandstones, with impressions and casts of the usual plants. 

 At 167 metres a bed of coal, somewhat impure, but yielding large 

 blocks, was come upon ; its dip was 41° S.* 



M. Breton explains these phenomena on the supposition that there 

 is here a great fold of the Coal-measures enclosed between strata of 

 Carboniferous Limestone, and that the Devonian strata pass over 

 them by means of a reversal, accompanied by a fault and thrust, which 

 has caused those beds to slide, as it were, over the Coal-measures 

 at a very small angle. Pig. 3 is a generalized section of this sin- 

 gular Coal-field reduced from the sections and particulars (omitting 

 some of the details) given in M. Breton's papers. 



One great point of interest which the Auchy-au-Bois Coal-field 

 presents is to be found in its relation to the probable extension of the 

 Coal-measures under the same Cretaceous strata in England, and the 

 light it throws upon the age of the Hardinghem Coal-field. 



Sir R. Murchison and Mr. Godwin- Austen considered this latter 

 coal-field, which lies between Calais and Boulogne, to belong to 

 a lower and more unproductive part of the Carboniferous series than 

 the Belgian and Yalenciennes field ; and this view I adopted in my 

 Beport in 1869. From subsequent examination of that coal-field, 

 I am now satisfied that the view taken by M. Gosselett is the correct 

 one, and in this my friend Mr. God win- Austen also now agrees. 



According to M. Gosselet, the Hardinghem Coal-field is a pro- 

 longation of that of Yalenciennes. He proves it both by the litho- 

 logical characters of the strata, by their organic remains, and by 

 physical structure. Later observations at Auchy-au-Bois also show 

 that the Coal-measures at Hardinghem are exactly on the prolonga- 

 tion of the strike (about E. 30° S., and W. 30° 1ST.) of the Auchy-au- 

 Bois trough ; though west of Auchy-au-Bois there is an apparent 

 thinning out of the fold that leads to the supposition that the coal- 

 strata are not continuous between the two places, but form separate 

 basins. 



The prolongation of the same strike and accompanying boundary 

 faults across the Channel would place the southern boundary of any 

 coal-field under the Tertiary and Cretaceous strata of South-east 

 England on a line which would pass a little south of Maidstone, 

 whence it would cross the Thames obliquely, and range a short dis- 

 tance north of London. It must be remembered, however, that 

 although such strikes and such faults may maintain a certain mean 

 average direction, they are liable to considerable deviations, so that 

 any underground Coal-measures, if there, may be either somewhat to 

 the north or to the south of this line ; but it is nevertheless on or near 

 this line that they should be first sought for. Their exact course 

 can only be determined by experiment. 



It is, in any case, of the highest interest to find that we have 



* At the time we visited the pit the works had been carried deeper, and they 

 were working a fine seam of coal, which in places was 4 metres thick. 



t " Etude sur le Terrain Carbonifere du Boulonnais," Mem. Soc. des Sc. Agr. 

 &c. de Lille, 3rd ser. vol. xi. 1873 : see also M. Gosselet's various papers in the 

 Annales de la Soc. Geologiqne du Nord. 



