50 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



were formed. I am satisfied however that the formation of every 

 variety of the Gibraltar breccia is a subaerial process ; the rain- 

 water, percolating through the fissures which everywhere intersect 

 the rock, dissolves a certain quantity, but as the largest portion of 

 the water is evaporated before it reaches the sea, it deposits the 

 calcareous matter upon whatever it comes in contact with, and 

 unites the whole into breccia, a process which cannot take place 

 under water; but if we admit that the breccia has been formed 

 upon land, we must also admit many and extensive movements of 

 depression, for we everywhere find it sea-worn. When the beds are 

 marine, they must have been elevated before they were cemented 

 into breccia, again depressed before they were water-worn and 

 scooped into sea-caves, and after that elevated many hundred feet 

 above the sea, as we find them near Martin's Cave. 



The next class of causes which have acted upon the rock of 

 Gibraltar are of a chemical nature, and to one of these I have ascribed 

 the formation of the breccia, namely the solvent quality of water 

 combined with a certain portion of carbonic acid ; and when we con- 

 sider the extraordinary quantity of breccia which everywhere covers 

 the flanks of the mountain, and the quantity of calcareous matter 

 required to cement it, which has been abstracted from its internal 

 recesses, we can have no difficulty in accounting for the number 

 and extent of its caverns. Several of these, as formerly noticed, are 

 external, and have been scooped out by the sea ; yet even these, 

 when we examine them, are generally found to be pre-existing fis- 

 sures widened. The sea-worn caves are horizontal and at right 

 angles with the line of coast ; the internal ones are vertical and 

 branch out in every direction: one of these, the well-known cave of 

 St. Michael, is of unknown extent, and from the recent researches 

 of Lieutenant Risk, R.N., appears to communicate with both sides 

 of the mountain ; on the east side, which is not obscured by vegeta- 

 tion, may be seen in inaccessible situations the openings of many 

 such caverns. 



The great mass of the breccia is composed of the fragments of lime- 

 stone, which have been thrown down or fallen to the base of the rock ; 

 the next in quantity is composed of the marine sandstone. It is to 

 be observed, that where these beds are not in a position to receive 

 the water from the limestone rock, they have not been converted 

 into breccia. This is the case with the extensive beds of sand on 

 the west side ; the water which falls upon them does not pass over 

 the rock, and they are still unconsolidated. 



Landslips. — At the base of the southern eminence we have proofs 

 of an extensive landslip ; the rock which has given way is limestone 

 breccia; the sides of the ravines are water-worn, showing change of 

 level since it took place. At the base of the North Front there has 

 been another landslip, also previous to the present sea-level, as the 

 mass of rock which has fallen down has been hollowed out all round 

 by the action of the sea. 



The inland cliffs and terraces at the southern extremity, as well as 

 the littoral caverns, which are numerous, and occur at every eleva- 



