THE GEOLOGY OF GIBRALTAK. 507 



to the east a bold escarpment, which is for the most part inacces- 

 sible, and in places almost vertical, the cliffs where they are lowest 

 having a drop of not less than 300 or 400 feet, and of more than 

 1000 feet where they approach the sea on the north. From their 

 base the ground falls rapidly away to the coast-line at angles that 

 vary from 30° to 40°. The opposite slopes of the dividing ridge are 

 not so abrupt, the only really precipitous portion that faces the west 

 being the line of cliff which overlooks Gardiner's Koad and Engineer 

 Road, between the Moorish Wall and the Mount. A low sandy 

 plain, that does not average more than 10 feet in height above the 

 sea, connects the Bock with the mainland. 



Rocks entering into the formation of the Promontory. 



The rocks of which the famous promontory is composed consist 

 of: — 1. Limestone; 2. Shale, with intercalated beds and bands of 

 grit and hardened mudstone ; 3. Limestone-agglomerate ; 4. Bone- 

 breccias ; 5. Calcareous Sandstones ; 6. Sand and loose debris. Of 

 these, 1 and 2 are probably of Lower Jurassic age ; the others are 

 all of much more recent date, being in fact mere superficial accu- 

 mulations. 



The strike of the limestone and shale undulates gently from 

 north to south, and thus corresponds with the trend of the pro- 

 montory. Prom the North Front to the termination of the great 

 escarpment the dip is westerly, generally at a high angle. In some 

 places, however, it is quite vertical, as in the neighbourhood of 

 Monkey's-Cave Road. Vertical dips of the shales and grits are also 

 well seen on the beach between Jones's Battery and Jumper's 

 Battery. In Windmill-Hill and Europa Flats the beds dip per- 

 sistently towards the east. 



More than three fourths of the promontory are occupied by the 

 basement rock, which is a greyish-white or pale grey, compact, and 

 sometimes finely crystalline limestone, arranged in more or less 

 regular beds that vary in thickness from a few feet to many yards, 

 the rock becoming in some places apparently amorphous, so that, 

 were it not for abundant jointing, monoliths of almost any dimen- 

 sions might be obtained from it. Frequently the bedding is very 

 obscure, and can only be detected when viewed in a good light from 

 some little distance. Here and there the rock has a striped or 

 banded appearance, the bands (which are usually somewhat darker 

 than the main mass of the rock) varying in thickness from an inch 

 or two upwards. In some places, also, thin ribbons of cherty 

 limestone and chert present themselves ; and layers and partings of 

 shale occasionally occur, but they are certainly not common. Now 

 and again the rock seems made up of angular and subangular grit 

 and small fragments of grey limestone, agglutinated together in a 

 paste of the same material, and looking not unlike some of the 

 coral-rock of modern reefs. In various places, especially along the 

 crest of the ridge and in the Europa district, the limestone is often 



