VOYAGE TO ALGERIA 177 



of the twist fading gradually and uniformly off, in passing 

 from south to north, the want of uniformity in the material 

 has produced lines of dislocation where there are abrupt 

 changes in the amount of twist. Thus, at the northern 

 end of the rock the dip to the west is nineteen degrees; 

 in the Middle Hill, it is thirty-eight degrees; in the centre 

 of the South Hill, or Sugar Loaf, it is fifty- seven degrees. 

 At the southern extremity of the Sugar Loaf the strata 

 are vertical, while further to the south they actually turn 

 over and dip to the east. 



The rock is thus divided into three sections, separated 

 from each other by places of dislocation, where the strata 

 are much wrenched and broken. These are called the 

 Northern and Southern Quebrada, from the Spanish 

 "Tierra Quebrada," or broken ground. It is at these 

 places that the inland caves of Gibraltar are almost exclu- 

 sively found. Based on the observations of Dr. Falconer 

 and himself, an excellent and most interesting account of 

 these caves, and of the human remains and works of art 

 which they contain, was communicated by Mr. Busk to 

 the meeting of the Congress of Prehistoric Archaeology 

 at Norwich, arid afterward printed in the "Transactions" 

 of the Congress. 1 Long subsequent to the operation of 

 the twisting force just referred to, the promontory under- 

 went various changes of level. There are sea-terraces and 

 layers of shell-breccia along its flanks, and numerous caves 

 which, unlike the inland ones, are the product of marine 

 erosion. The Ape's Hill, on the African side of the 



1 In this essay Mr. Busk refers to the previous labors of Mr. Smith, of 

 Jordan Hill, to whom we owe most of our knowledge of the geology of the 

 rock. 



