THE GEOLOGY OF GIBRALTAR. 519 



steep surface. The rain that does not find its way into fissures and 

 caves rushes at once down the slopes to the low grounds. It has 

 no opportunity of collecting into a few channels, so as to pour in 

 one heavy torrent of depth sufficient to sweep blocks weighing 

 several tons for a distance of about 600 yards down an incline of 

 less than 1 in 7. Small angular fragments, indeed, and now and 

 again larger stones (did they chance to lie in a trench or gully) 

 might be carried away during a sudden shower of heavy rain ; but 

 it is idle to suppose that the great agglomerates which cover nearly 

 all the districts of Eosia and Buena Yista could have originated in 

 this way. The cones de dejection formed at the base of the Eock at 

 the mouths of the one or two gullies which at present carry occa- 

 sional torrents to the low grounds are utterly unlike any of the 

 agglomerates we are now considering. They consist chiefly of sub- 

 angular stones, mostly of small size, few measuring more than 

 6 inches across ; they are of small width and length, and do not 

 extend outwards much beyond the foot of the steep acclivities of the 

 Eock, and they are decidedly coarsest towards their upper limits. 

 Moreover, in such cones de dejection stratification, however rude it 

 may be, is usually present ; but in the great limestone-agglomerates 

 not only are the stones all more or less sharply angular and 

 arranged generally without any reference to size or figure, but the 

 agglomerate at the foot of the Eock is not sensibly coarser than that 

 which occurs at Eosia Bay, a distance of 500 or 600 yards away, 

 while traces of bedding are very exceptional indeed. 



Eeflecting upon these and other considerations, we are led to 

 infer that the great agglomerate of Eosia and Buena Yista is the 

 record of a much more wintry climate than we could have imagined 

 as likely to have characterized the Eock in recent times. We seem 

 to have, in that agglomerate, evidence not only of the former action 

 of frosts severe enough to wedge out large blocks of limestone, but 

 also of more or less sudden meltings of thick snow, such as would 

 saturate heavy trains of debris and cause them to move en masse 

 down the steep slopes and over the lower grounds beyond. Under 

 such conditions some portions of the debris could hardly fail to be 

 partially arranged by the action of torrential waters, so as to give 

 rise to the appearance of bedding which is noticeable here and there 

 at Buena Yista. 



The highly denuded aspect of the agglomerate implies a long 

 lapse of time since the period of its formation. It probably at one 

 time cloaked the lower slopes of the Eock as a more or less con- 

 tinuous sheet, which may have extended far up the steeper acclivities, 

 although these are now quite bare of either debris or agglomerate. 

 Shelves, terraces, and deep gullies have been excavated in the 

 agglomerate, and it everywhere so clearly bears the marks of having 

 suffered extreme denudation that there is no difficulty in believing 

 that it may originally have clothed the Eock up to near the summit 

 at O'Hara's Tower. The dip of the agglomerate at Buena Yista, 

 which is towards instead of away from the Eock, seems to indicate 

 that the mass thereabouts may have been undermined by water 



