WILLIAMS ON THE TRAP-ROCK OF BLEADON HILL. 



49 



the beds of limestone diagonally, overhanging on its southern side. 

 Close to the northern side of this mass, a well-defined joint 

 cuts the limestone from its base to its summit, so that between 

 this joint and the trap there is a thin strip of limestone which, on 

 its side next the trap, is very irregularly indented. 



Section II. 

 Eastern side. 





Proceeding next to the eastern side of the cutting, and beginning 

 at its northern end, the lias (b) is first seen, dipping towards 

 the wall of limestone and trap. On the opposite side of the 

 fault, and at the base of the limestone is seen a mass of trap (p'), 

 the upper surface of which runs nearly parallel to the calcareous 

 beds above it. Further southward, however, that surface becomes 

 irregular, and descends more rapidly than the beds of limestone ; 

 and near its southern end the mass of trap descends at an acute 

 angle below the level of the cuttings. Further southward, a 

 second mass of trap appears, and at a short distance a third, and 

 then finally a fourth. 



The author next describes the alterations produced in the lime- 

 stone on the eastern side of the cutting. 



At the northern fault, the face of the wall of limestone that lies 

 above the trap is eroded in places into broad shallow cavities 

 filled with trap and trappean matter, the trap having generally 

 selected the joints and fissures in the limestone, and being therefore 

 variable and unequal in thickness here, as on the other side of the 

 cutting. 



Three of the beds of limestone have been affected by the in- 

 trusion of the trap : the lowest varies much in thickness, and is 

 reduced where the trap appears. The second bed is affected and 

 sometimes cut off by the trap, and sometimes becomes blended 

 with the superior bed of limestone, and towards the fault dips 

 down towards it. This latter bed abuts against the fourth mass 

 of trap (towards the south), the contact being rough and rugged. 



The curvilinear fault above noticed has shifted the beds of 

 limestone. On the southern side of this fault, we find resting on 

 the southern flank of the fourth mound of trap a mass of altered 

 limestone, which, as it approaches the summit of the trap, tapers 

 upwards and disappears. The surface of this wedge of lime- 

 stone, where in contact with the trap, presents the same pitted and 

 irregularly eroded appearance which has been before noticed in 

 limestone similarly situated. 



VOL. I. E 



