48 PROCEEDTNGS OF THE GEOLOGICAL SOCIETY. 



niucli obscured by sands, breccia and vegetable soil, as to prevent 

 any satisfactory conclusions being arrived at. 



The next class of movements to which the rock has been sub- 

 jected is that producing general change of level. No such change 

 has taken place during the historical, or probably the human period. 

 Ancient geographers describe it as almost surrounded by the sea ; 

 the ruins of Carteia, a city of antiquity in the time of the Roman 

 republic, can be traced to the level of the sea ; and ancient graves, 

 containing stone hatchets and daggers, have recently been disco- 

 vered not more than ten feet above it. 



The sandy plain immediately to the north of the Fortress appears 

 to have belonged to the period of stationary level which imme- 

 diately preceded the present : it is full of marine shells. On the 

 British side of the Spanish lines the ground has been much dis- 

 turbed by the operations of the different sieges, but beyond them it 

 is in its natural state, and affords a most perfect example of a raised 

 shallow sea-bottom. On the west side it is buried under sand-hills, 

 but on the east, where it is exposed to the winds from the Mediter- 

 ranean, the sands have been blown away, leaving the deserted 

 shells on the surface in such numbers, that when seen from the 

 summit of the rock the ground is absolutely white with them. 

 Notwithstanding their numbers, however, on my first examination I 

 could not find more than five or six species, and these exclusively 

 bivalves, the most numerous of which were the Cardium tubercu- 

 lare^ Pectunculus pilosus, Donax trunculus, and a Venus allied to 

 Venus gallina, the V, senilis of Philippi, An examination of the 

 sandy bottom of the bay by the dredge showed that it also was the 

 exclusive habitation of gregarious bivalves. The Spanish boats 

 dredge for shell-fish in the adjoining bay in about two-fathoms 

 water : in examining the cargo of one of the boats, I found but one 

 univalve, which contained a hermit crab, as if to prove that it was a 

 straggler. The species were the same as those of the raised deposit ; 

 the only difference I could observe was, that the thick and strong 

 shells, the Cardium and Pectunculus, were more abundant in pro- 

 portion than in the ancient deposit ; but this is easily accounted for, 

 since these shells live in agitated water, and though they abound on 

 the Mediterranean side of the neck of land, they are rare on the 

 shores of the bay side ; but when it was covered by the sea it was 

 unsheltered, and naturally contained both kinds of mollusks. No- 

 thing, indeed, can be more perfect than the resemblance of this 

 ancient sea-bottom to that of the present bay, or, rather, to the ap- 

 pearance which it would present if raised above the sea, since 

 the eastwardly winds from the Mediterranean would then blow the 

 sands to the west side, and leave the shells on the surface. As 

 the boats dredge in two-fathoms water, and this shelly deposit is 

 raised about two fathoms, the difference or amount of the change 

 must have been about 24 feet. Now it is remarkable that ex- 

 actly 24 feet above the present littoral zone an ancient one was 

 discovered, in which the resemblance was, if possible, still more 

 perfect than in the corresponding sea-bottoms. The rocky shores 



