GEOLOGICAL OBSERVATIONS IN SOUTH GEORGIA. 803 



lying them. The colour is clue to the oxidation of ferrous iron in the rocks to the 

 ferric state. The colour is superficial, as the rocks are greyish on a fresh fracture, 

 They are generally crumpled and indurated. They are highly siliceous, and while 

 some of them are fine-grained others are more or less gritty. Interbedded with them, 

 in places, are still coarser rocks which may be regarded as fine conglomerates. There 

 are also interbedded black shales which are not of great thickness in any one body. 

 There are also beds of volcanic tuff. 



The black shales are finely fissile, and slides cut from them show a cherty 

 structure with usually a carbonaceous and fine-grained matrix. The shales, although 

 black when freshly fractured, take the rusty-brown colour of the great mass of the 

 rocks with which they are interbedded, so that their presence is generally obscured 

 in the numerous exposures on the north-east coast and in the adjacent glens and 

 fiords or gorges. 



The rusty -brown rocks are not only the best horizon for determining the strati- 

 graphical succession of the various rock-forming bodies in the island, but their pro- 

 nounced colour differentiates them sharply from the rocks above and below them. 

 They are also useful in giving us an idea of the total rock thickness. There are compact 

 masses of these rocks at Leith Harbour and Port Gladstone, the latter rising to an 

 exposed thickness of about 1500 feet, and resting on an exposure of 133 feet down to 

 sea-level of the rocks underlying them. The exposure is in the steep sloped hill at 

 the head of Port Gladstone Harbour (PI. XC, fig. 2). 



A much greater thickness is exposed in Moraine Fiord, Cumberland Bay, and in 

 the rocky escarpments on the shoulders of Mount Paget. There is an apparently 

 continuous body of these rocks, rising to the top of the wall-like escarpment shown 

 in PI. LXXXVII, fig. 2. As there is no break in continuity with the rocks under- 

 lying them, only a change of colour, we have from the sea-level in Moraine Fiord 

 to the top of the big escarpment a thickness of 4200 feet. 



Unless there is some concealed line of faulting, that thickness is entirely composed 

 of stratified rocks in regular and unbroken succession. As the rocks underlying the 

 rusty-brown rocks rise to a considerable height in Moraine Flat and the north-west 

 side of Moraine Fiord, a part of the total thickness must be attributed to them. 

 A rough approximation is about 3000 feet of rusty-brown rocks and 1200 feet of the 

 lower rocks. 



Cumberland Bay Series, Loiver Division {Dark Shales, Greywackes, 



Tuffs, Gritty Shales, etc.). 



These rocks are the downward continuation of the rusty-brown rocks, without 

 an apparent unconformity. They are much the same in texture as those above them, 

 but dark fissile shales reach considerable thicknesses in this division, and collectively 

 are the predominant rock. They alternate with gritty and fine-grained arenaceous 

 shales, with coarser arenaceous rocks which may be regarded as greywacke, and with 



