Geological Society of London. 183 



stratification as the rock below. The islet known as Gunner's Quoin consists of co- 

 lumnar basaltic lava, capping volcanic sand, below which is a browner volcanic sand 

 with seams of coral fragments. Flat Island is in part the remains of a volcanic 

 crater, and the rest consists of volcanic sand strewn with coral blocks. There are 

 basaltic dykes in the hill, the top of which appears to show traces of one or more 

 plugs. The author concludes that Mauritius was once an active volcano, now 

 elevated with the old reef. The islets also formed part of a volcano or volcanos, 

 and have also been elevated with reef -material. 



IV.— March 6tli, 1878.— Henry Clifton Sorby, Esq., F.E.S., 

 President, in the Chair. — The following communications were read : 



1. "On the Geology of Gibraltar." By Prof. A. C. Ramsay, LL.D., F.E.S., 

 and James Geikie, Esq., LL.D., F.R.S. 



In this paper the authors, after giving some account of the physical features of 

 Gibraltar, described in detail the various rock-masses of which the peninsula is 

 composed. The chief rock is a pale grey, bedded limestone, overlain by shales con- 

 taining beds and bands of grit, mudstone, and limestone. Fossils are very rarely 

 met with in the limestone, and have never as yet been found in the shales. The 

 only recognizable fossil they obtained from the limestone was a Ehynchonella, which 

 Messrs. Etheridge and Davidson think is most likely Eh. concinna. This would 

 make the beds of Jurassic age. The limestone forms the great eastern escarpment, 

 and dips west under the shales, which form the lower slopes upon which the town is 

 built. The dips vary from 12" or 20° up to vertical. The connexion of these strata 

 with the rocks of the adjoining districts in Spain and the opposite coast of Africa 

 was traced, and it was shown that the Gibraltar Limestone reappears in Ape's Hill 

 in Barbary, while the overlying shales and the sandstones of Queen of Spain's Chair 

 form all the ground to the west of Ape's Hill up to Cape Spartel. The Jurassic 

 strata of Gibraltar are overlain by various superficial accumulations, the oldest of 

 which is a great mass of limestone-agglomerate, which is unfossiliferous, and shows 

 as a rule no trace of stratification. It is made up of angular blocks of limestone of 

 all shapes and sizes, and rests upon an rmeven surface of limestone ; it also covers 

 wide areas underneath which only shales are present. It is excessively denuded, 

 being worn into ravines and gullies, and presents generally a highly honeycombed 

 surface. Terraces of marine erosion have also been excavated in it. It is not 

 now accreting, and could not have been formed under present conditions of climate 

 and surface. The authors gave at length their reasons for believing it to have been 

 the result of a severe climate. The blocks were wedged out by the action of frost, 

 and the heaps of angular debris thus formed were saturated by water derived from 

 melting snows, and so were caused to flow en masse down the mountain-slopes and 

 over the gently-inclined ground at their base. 



The caves and fissures of Gibraltar were then described. It was shown that the 

 true bone-breccias were confined to these. Many of these fossiliferous breccias are 

 of later date than the great agglomerate, since they are met with in fissures and 

 caves that intersect the limestone and limestone agglomerate alike. When the 

 mammalia tenanted Gibraltar, Africa and Europe were united, and the climate was 

 genial. 



All round the rock occur platforms, ledges, and plateaux, which are evidently the 

 work of the sea. These erosion-terraces are covered in many places with calcareous 

 sandstones containing recent species of Mediterranean shells. Such marine deposits 

 occur up to a height of 700 feet. The movement of depression was interrupted by 

 pauses of longer or shorter duration, and the climatic conditions were probably much 

 the same as at present. 



After the rock had been re-elevated, the subaerial forces modified the surface of the 

 marine sands that covered the limestone platforms, so that they came to form long 

 sand slopes. The land at this period was of greater extent than it is now, and some 

 grounds exist for believing Europe to have been again united to Africa, for mamma- 

 lian remains occur here and there in the deposits that overlie the limestone-platforms. 

 These relics, however, it is just possible, may be derivative. The climate was prob- 

 ably still genial like the present. 



Overlying the marine and subaerial deposits just referred to occurs an upper and 

 younger accumulation of massive unfossiliferous limestone agglomerate. This 

 deposit the authors believe to owe its origin to severe climatic conditions. After the 



