520 A. C. RAMSAY AND J. GEIKIE ON 



acting along the line of the main fault. A very slight sinking in of 

 the ground along the line of that fault would account well enough 

 for a reversal of the dip of the agglomerate, which, under any view 

 we take of the origin of that accumulation, could only have been 

 originally away from the Hock. 



2. Bone-breccias in Caves and Fissures. 



Although we visited a number of the caves, yet the object of our 

 journey to Gibraltar did not allow us to make any special and 

 detailed examinations, so that we have but little to add to what has 

 already been said about the caves by previous observers. These 

 caves are of two classes, as Mr. Busk and Dr. Falconer have pointed 

 out, namely, 1st, those which have horizontal pavements, and which 

 appear upon the face of old sea-cliffs ; and 2nd, those that descend 

 from the surface at various angles into the rock. The former have 

 evidently received their present form from the action of the sea, 

 although it is probable that in many cases they already existed 

 before the sea got access to them. The latter, again, there can be 

 no question, owe their origin to underground waters, their direction 

 having been determined by the joints and fissures and bedding- 

 planes of the limestone. Some writers have supposed that these 

 caves, which often descend at a very high angle, sometimes ap- 

 proaching vertically, are merely gaping fissures formed during 

 violent rupturing of the strata ; but the faults observed by us in 

 actual section afforded no evidence of having been left as wide, open 

 fissures. Yet one may readily understand how such faults, by 

 intercepting the water that descended from the surface by joints 

 and bedding-planes, would tend to divert much of the drainage in 

 one particular vertical direction. Faults would thus merely play 

 the part of great master-joints, and become gradually widened by 

 the chemical and mechanical action of subterranean water. Many 

 of the caves, however, do not occur on lines of breakage, some of 

 them having been excavated along lines of jointing, while others 

 evidently coincide with planes of bedding, as, for example, those 

 which are seen in the old sea-cliff below No. 4 Europa Advance 

 Battery. 



Often enough fissure-breccias which are now exposed at the 

 surface appear to lie in cavities or trenches of erosion which have 

 not always been so open to the day. In many places, again, they 

 seem to fill up mere superficial hollows, which terminate down- 

 wards. It is from such fissure-breccias that mammalian remains 

 appear to have been frequently obtained. Similar surface-fissures, 

 indeed, are in process of forming and being filled up even now. 

 This is well seen on the steep slopes above the town, where the 

 honeycombed and weathered aspect of the limestone is very con- 

 spicuous. Bain is gradually widening the joints by dissolving out 

 the limestone and leaving behind a red earthy residuum. Irre- 

 gular-shaped fragments thus become loosened, and may fall out of 

 place or be forced away by winter torrents, and swept,* along with 



