4-4? PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



success than the discovery of rude fragments of pottery, a piece of 

 melted lead, and the bones of recent animals. 



The subordinate beds of chert present no peculiarity which re- 

 quires to be noticed, and there has been no coal found in situ upon 

 the rock itself. I shall now proceed to inquire into the geological 

 causes to which the rock owes its present configuration. 



Its geological history is instructive as an example of the extraor- 

 dinary number as well as complicated nature of the movements which 

 have caused the irregularities on the surface of the earth. We are 

 apt to ascribe the presence of marine remains in elevated situations 

 to some great convulsion by which they have been raised from the 

 bottom of the sea, and to suppose that they have remained subjected 

 to no changes but such as are caused by the diurnal action of the 

 elements ; but an examination of the mountain of Gibraltar forces us 

 to admit that it must have undergone not one but many movements 

 both of elevation and depression, some of them attended with rup- 

 ture and dislocation of the strata, others with mere changes of level ; 

 to these must be added the effects of chemical agents, landslips, 

 blown sands, and the wasting action of the sea. Some of the dis- 

 turbances must have obliterated every trace of preceding ones; but 

 although we cannot hope to be able to describe all the changes which 

 have taken place, enough of evidence remains to attest the existence 

 of niany of them, and to furnish a clue by which we can in some 

 measure decipher the order of their occurrence. 



We know from the marine remains contained in them that the 

 beds of limestone must have been formed at the bottom of the sea 

 during the secondary period in a position nearly horizontal, and that 

 they must have been lifted up to an angle with the horizon by some 

 force acting from below, doubtless of an igneous nature. Whatever 

 it was it must have been deep-seated, and there are no igneous rocks 

 in the neighbourhood. 



A force sufficient to rend the superincumbent beds would tilt them 

 upwards on one or both sides of the rent, and if any part of them 

 stood above the surface it would assume the form of a three -sided 

 prism, one of whose sides coincided with the planes of the beds, an- 

 other with their truncated edges ; the third side, or apparent base, 

 would be formed by the surface of the sea, whilst the ends of the 

 prism would be nearly perpendicular. It would appear in the case 

 of Gibraltar that the rent or fissure caused by the subterranean force 

 ran south and north ; that the beds gave way on one side of the rent 

 (the west) and were lifted up to an angle of about 19°. From the 

 natural tendency which hard flat bodies have to break across at right 

 angles, the inclination of one of the slopes being 19°, that on the 

 opposite side of the ridge would be about 71°, the complement of 

 the right angle, whilst the ends would be perpendicular. 



If we raise the front edges of an octavo volume about an inch and 

 a half off a table, leaving the back resting upon it, and suppose it 

 nearly submerged in water, the part above the surface would afford 

 a tolerably correct model of what the rock must have been after its 



