SMITH ON GIBRALTAR. 45 



first Upheaval. The boards would be found to form an angle of 

 about IQ"^ with the surface, the front edges 71°, whilst the ends would 

 be vertical. 



This, if not the original form, is at all events the most ancient form 

 of the rock of which any evidence remains. In its northern extremity 

 we have the ruins of what must have been one of the ends of the 

 great prism, and in the stupendous escarpment of the North Front 

 its vertical termination. 



The earliest epoch in the history of the rock is that in which its 

 beds were deposited at the bottom of the secondary ocean. The 

 second is that which followed the first upheaval, in which it took the 

 form already described. In this position fresh beds would be de- 

 posited round its base composed of the disintegrated matter of the 

 rock itself and that of the neighbouring shores. 



a. Elevated limestone rock. b. Sea forming new deposits at its base. 



Accordingly we find in different parts of the rock, and chiefly in 

 the presence of raised marine deposits which must have been de- 

 posited subsequently to the first upheaval, but anterior to the second, 

 proofs not only of the changes of level which it must have under- 

 gone during this period, but also of the long duration of the period. 

 In process of time however a second upheaval has taken place which 

 tilted up the beds l^° more than they were at first: ihh disturbance 

 however did not extend to the whole peninsula, the northern part 

 being left in its original position : the rock must therefore have been 

 broken across, and the line of fracture must be that which separates 

 the beds in the original position with a dip of 19° from that part 

 which has been elevated 19° more, and is consequently at an angle 

 of 38°. This separating line is well-marked by the gap in the sum- 

 mit and ravines on the sides which separate the Middle Hill from 

 that on the north or Rock-gun Height (see first diagram in page 4-'/). 



The consequence of this movement has been to place the oldest 

 or limestone beds at the inclination of 38°, in which we find them at 

 Middle Hill, and the new beds subsequently formed at an angle of 

 19°, or sloping inwards to the hill, as may be observed at its bas(\ 

 During the long continuance of the preceding epoch, the wasting 

 action of the sea had scooped out a cliff and terrace : upon this tcr- 

 race horizontal beds of sand had been deposited, and upon these, 

 again, newer beds have been deposited unconformably or sloping 

 outwards. Finally, in one of the numerous changes of level to wliicii 

 the rock has been subjected, and of which proof will be afterwanis 

 offered, the whole of thcvse beds have been lifted up and covercfl 

 with the blown sands of Catalan Bay on the east side. These sands 

 slope against the face of the cliff at an angle of 30°, at which incli- 



