THE GEOLOGY OF GIBRALTAR. 521 



red earth, loose stones, and organic debris, into sheltered nooks and 

 crevices, where in time all may become hardened into a solid mass 

 by infiltration. It is worthy of note, however, that the loose stones 

 and fragments which are found lying here and there over the 

 surface of the rock, or gathered together into long trains of debris 

 by the winter rains and torrents, are not, as a rule, so sharply 

 angular as the bulk of the stones that occur in the unfossiliferous 

 limestone-agglomerates. 



In the caves the breccia is of a dark red colour, and appears to be 

 made up chiefly of bones set in a matrix of red earth and hardened 

 by calcareous matter. We saw very little of it in place, however, 

 and cannot therefore say what proportion of limestone fragments it 

 may contain. 



In regard to the age of the caves and fissures, it is of course im- 

 possible to say in all cases how old these cavities may be ; but there 

 is indubitable evidence to show that some of them, and these the 

 best known and most important, are of later date than the great 

 agglomerate of Buena Vista. The famous bone-breccia at Bosia 

 Bay, for example, clearly occupies a vertical fissure of erosion in the 

 unfossiliferous limestone-agglomerate. From this fissure tori-loads of 

 mammalian remains were obtained during the scarping of the cliffs, 

 but unfortunately these have long been dispersed. To approxi- 

 mately the same date belongs the Genista Cave with its bone- 

 breccia, as will be shown presently. All these caves and fissures 

 have undoubtedly been filled by the washing into them of materials 

 lying loose at the surface. It is evident, however, that since the 

 time when the mammalian remains and limestone- debris were swept 

 into the caves and fissures, the surface of the ground has been 

 greatly modified. The fissure at Bosia, for example, now traverses 

 a nearly isolated hill or craggy knoll (9th Bosia Battery) ; but when 

 water flowed through it the agglomerate evidently could not 

 have presented the same highly eroded appearance. When the 

 bones were being conveyed into the Bosia fissure and the Genista 

 Cave, the configuration of that part of the Bock must have differed 

 very much from the present. 



3. Raised Beaches, Calcareous Sandstones, Sfc. 



The promontory of Gibraltar is marked in various places and at 

 different levels by terraces or platforms cut into the solid rocks, and 

 backed of course by cliffs which are more or less precipitous. Upon 

 these platforms rest deposits of calcareous sandstone and limestone- 

 agglomerate, which often entirely conceal the underlying horizontal 

 or gently inclined pavements. The deposits referred to are thickest 

 immediately under the cliffs, from the base of which they slope 

 gradually outwards. The character of the rock-platforms, and the 

 nature of the sandstones which lie upon them, leave one in no doubt 

 as to the marine origin of both. 



The most recent marine deposits are those which occupy the low 

 isthmus that connects the Bock with the mainland. The surface of 



