224 GENERAL GEOLOGY 



Discovery, adjacent to it, 9085 feet; Mount Erebus, over 13,000 feet; Mount 

 Terror, 10,755 feet; and Mount Melbourne, 8000 feet. It is evident from these 

 altitudes that if this volcanic chain is a continuation of that of the South American 

 Andes the extrusive terrestrial forces have here in the Antarctic less elevating 

 power than they have in the South American Andes, where several volcanic cones 

 exceed 20,000 feet in altitude. 



DELLBRIDGE ISLANDS 



These four islands lie 3 or 4 miles from the nearest point of Ross Island, with 

 their longest axes directed nearly east and west. They rise steeply out of deep 

 water, and the steepness of their slopes continues above water, so that they are 

 all of the razor-back type. They consist of volcanic rock of a number of types, 

 but on the whole allied to those of the adjoining mainland of Ilo.ss Island. 



Inaccessible Island. Inaccessible Island is about 450 feet high, very steep 

 towards the south, but sloping sufficiently on its northern side to allow of the 

 weathered material lying on its slopes, which are therefore covered by a mantle 

 of angular debris. Our information about the island is very meagre, as it was 

 mainly gathered during the initial or final stages of different sledge-journeys, when 

 time and food considerations did not allow of any length of stay. 



Its name. Inaccessible Island, given by Captain Scott of the National Antarctic 

 Expedition, 1901-4, is very appropriate, for the first effect of the summer thaw 

 is to melt the sea ice immediately round its ice-foot, and thus cut off the island in 

 summer, so that it was only during spring and early summer that it was accessible 

 for om- sledge parties. Mi'. Hodgson, biologist on the Discovery, collected specimens 

 which proved to be red vesicular trachyte, jiorphyritic basalts, yellow trachytes, 

 and a trachy-dolerite of intermediate character. 



On August 23, 1908, Sir Ernest Shackleton walked from our winter quarters 

 at Cape Royds to the island, and brought back specimens which were identified 

 from their microscopic characteristics as trachytes and tuffs. The trachytes he 

 obtained in situ, and reported it as occurring as a flow beneath the basalt, thus 

 adding one more link to the chain of evidence regarding the relative ages of 

 trachytes, basalts, and kenytes. 



The eastern portion of the island was examined by a Hut Point sledge-party on 

 September 4th, and proved to be formed of a sheet of massive basalt overlying a red 

 basaltic agglomerate with many pockets of yellow ochreous dust. Farther to the 

 north were several blocks of a reddish kenyte with platy felspars, which appears to 

 be intermediate in character between the rocks of the Turk's Head and those of 

 Cape Royds, but inclined to be rather more like the Turk's Head kenyte (1797). 

 One specimen secured contained a nodule or segregation of kenyte of a more acid 

 type with numerous felspars (1787). These blocks wei'e lying on the ice-foot, and 



