512 A. C. RAMSAY AND J. GEIKIE ON 



Bay is southerly, and at and near Algeciras more or less easterly ; 

 but the strata are now and again much contorted and confused ; the 

 prevailing dip, however, along the west coast of the bay appears to 

 be a little west of north. There can be no doubt that, underneath 

 modern superficial marine deposits, the bed of the bay is occupied by 

 sandstones and shales, similar to those of the Spanish ground just 

 described and those of the west coast of the Rock of Gibraltar. 



North of the Sandy Plain the strata of the Steeple Chase, the 

 Queen of Spain's Chair, and the neighbouring country consist, as 

 we have said, of thick beds of sandstone, with interstratified beds of 

 shale, which dip chiefly westerly, with some contortions, and 

 finally, east of Pachon, pass under the sands of the plains, appa- 

 rently with a southern dip, which, however, may be only local. 



The question thus arises, Do these sandstones extend beneath the 

 sands of the plain towards the limestone rock of the North Front ? 



By reference to the geological map (PL XXIII.) it will be seen that 

 the strike of the limestone and overlying shale gradually curves in 

 the neighbourhood of Grand Casemate Square ; for whereas south of 

 the Square the strike runs from south to north, beyond the Square 

 along the Inundation the strike of the strata bends round in a 

 north-easterly direction. This change of strike has been proved by 

 actual observation along the south-eastern side of the Inundation ; 

 but beyond that, across the Racecourse, nothing further is known. 



If, however, the strike of the shale and limestone be continued 

 on the assumption that they partly occupy the ground beneath the 

 superficial sands, then the Cemetery and the ground further east, to 

 the shore, a little further than the north-east corner of the British 

 lines, will be underlain by limestone, and part of the territory 

 immediately to the west and north underneath the sands will consist 

 of shales and intercalated calcareous grits &c, similar to those that 

 lie on the west side of the limestone of Gibraltar itself. 



It may, however, be questioned whether the limestone does 

 extend northwards to any great distance beyond the bold cliff of 

 the North Front ; and, after much deliberation, we thought it most 

 probable that a large fault, running about 23° north of east, cuts off 

 the limestone along the line, roughly speaking, of the Devil's Tower 

 Road, and passes into the bay somewhere about the Stone Jetty on 

 the western shore of the Isthmus. (This supposed fault is shown 

 on the map by a dotted line.) If this be so, then the fault is a 

 downthrow on the north ; and it is quite possible that it may be of 

 sufficient amount to throw the limestone, which underlies the 

 shales, so far beneath the surface, that the sandstones and shales of 

 the Spanish hills may underlie the sands of the Isthmus and abut 

 upon the limestone of the Rock on the southern side of the fault. 

 Considering the excessive disturbance of the strata of the Rock 

 itself, and the phenomena attendant on the Great Main Fault that 

 traverses it south of O'Hara's Tower, and also the intense disturb- 

 ance and contortion of the strata north and west of Gibraltar Bay, 

 such a fault as that inferred beneath the sands of the North Front is 

 not more remarkable than some of the phenomena visible elsewhere 



