GREENSAND AND PALEOZOIC ROCKS UNDER LONDON. 905 



This is especially the case on the south side of the great Coal-trough 

 iu the Liege, Mons, and Valenciennes Coal-fields. The Coal-measures 

 are, all through these districts, greatly disturbed, thrown back on 

 end, and not unfrequently reversed by the great disturbing axis of 

 the Ardennes and Artois*. 



A most remarkable instance of such a reversal has been recently 

 brought to light in the Coal-works at Auchy-au-Bois near Lillers. 

 As this is the most westerly point to which the Valenciennes coal- 

 field has been traced under the Tertiary strata and the Chalk, and is 

 the part of the basin nearest to England, I will briefly describe the 

 circumstances of its disco very f . 



Some years since the Coal-measures had been found to extend to 

 near Bethune ; but the many borings made further westward in 

 the " Pas de Calais " were stopped either by the Mountain Limestone 

 or by Devonian strata, so that further attempts were discouraged. 

 In two borings at Auchy-au-Bois this was also the case ; but the 

 geological inferences from stratigraphical structure and organic 

 remains were such as to lead the French geologists and M. Breton, 

 the manager of the Company at that place, to believe that there was a 

 reversal of the strata such as to render it possible that the younger 

 strata might be found underlying the older. 



Instead, therefore, of stopping the work on meeting with strata 

 older than the coal, it was in this instance determined to continue. 

 The result was as remarkable as it had previously been unexpected. 

 In one boring (No. 15, fig. 2), the Devonian strata were reached at 

 a depth of 131 metres, and at a depth of 168*30 metres the bore-hole 

 passed, as was anticipated, into the Carboniferous Limestone. With 

 the uncertainty, however, as to what might be the thickness of this 

 formation, the bore-hole was not carried beyond a depth of 215*11 

 metres; but another bore-hole (No. 16, fig. 1) was commenced at 

 a short distance to the north on the rise of the strata, and there, 

 under the Cretaceous strata, at a depth of 148 metres, the Car- 

 boniferous Limestone was reached ; at 170 metres the boring passed 

 out of that formation into true Coal-measures, in which, at 185 

 metres, the bed of coal (o) was met with. 



Another boring (No. 17, fig. 3) was then made further north, 

 and there the Coal-measures were reached immediately beneath the 

 " Tourtia," at a depth of 146 metres. A shaft was then sunk between 

 the last two borings. As was expected, the Carboniferous Limestone 

 was met with at a depth of 146*44 metres, which by its fossils 

 (Spirifer mosquensis, Aihyris Moissyi, Rhynchonella pentatoma, &c.) 

 M. Breton identifies with the lower division of that formation — the 

 " Calcaire de Tournay." These beds had a dip of 33° south, and ex- 

 tended to the depth of 155*40 metres, when a very acute fault was 

 traversed and the Coal-measures reached — the latter consisting of 



* See the sections given in my report "On the Probability of finding Coal in 

 the South of England," &c, p. 161. Royal Commission^ on Coal Supply, 1869. 



t This has been described by M. Breton in his " Etude sur le prolonge- 

 ment au sud de la zone houillere du Pas-de-Calais," Annales de la Societe Geo- 

 logique du Nord, tome iv. p. 138, 1876-77, and his ' Etude stratigrapbique.' 



