2 1 8 PREHISTORIC E UR OPE. 



ever their origin may be, they are not now accumulating. 

 On the contrary, since the time of their formation they have 

 experienced very considerable "denudation;" they have been 

 furrowed and worn and trenched by rain and torrents, in 

 precisely the same manner as the limestone from which they 

 have been derived. And not only so ; but the sea during 

 some period of submergence has carved out horizontal ter- 

 races in them which are continuous with similar ledges ex- 

 cavated in the highly-inclined calcareous strata. The breccias 

 are obviously of subaerial origin, but they indicate the former 

 existence of conditions very different from the present, for they 

 can only be the result of severe frosts. It is evident that 

 such were needed to wedge out the larger blocks that occur in 

 the breccia, some of which measure several yards in diameter, 

 and must weigh 20 or 30 tons at least. Moreover, the sharply- 

 angular shape of the stones is further proof that these have not 

 been subjected to the action of torrents or the sea. There was 

 a time, then, in the geological history of Gibraltar when the 

 winters were so severe that the limestone-beds were ruptured 

 and shattered, and the slopes of the Eock became covered over 

 with sheets of loose, angular debris and large blocks. But some 

 force other than frost was needed to carry this debris down the 

 gentler slopes, and to spread it over the low grounds that 

 extend outwards to the sea. Hard frosts might no doubt dis- 

 integrate the limestone, and scatter the fragments over the 

 steeper slopes ; but the impetus with which these rolled down- 

 wards would not suffice to carry them across the low grounds, a 

 distance of 550 yards at least, over which the average inclina- 

 tion of the ground is not more than 8° or 9°, while in some 

 places the slope does not exceed 2° or 3°. What appears to be 

 the true explanation of the phenomena may be observed in 

 most alpine regions, where hard frost and heavy snowfalls 

 occur. If we suppose that in former times thick snow mantled 

 the slopes of the Eock, we shall have all the conditions neces- 

 sary for the origin of mck-debris, and its translation over the 

 low grounds to what is now the sea-bottom. The limestone 



