198 GENERAL GEOLOGY 



rest of the rock. Several of these round nodviles had weathered out and were lying 

 at different distances from the parent rock, and when one or two of them were 

 collected, and broken across the middle with a hammer, they were seen to consist 

 of a very hard and indurated outer shell from an | to a i inch thick, enclosing a 

 dark-green ferruginous-looking sandstone of larger grain, and with less cohesion 

 between the grains. The mode of origin of these pot-holes was now clear, for they 

 were to be seen in all stages of formation, from the nodule which was left intact and 

 only jwlished on the outside, through the various intermediate stages, to the perfect 

 cup, where the inside had been completely cleaned out, and only the outer polished 

 shell was left, containing many of the quartz grains which had been used by the 

 wind as files to get rid of the less resistant portion of the stone. Even a further 

 stage was to be seen, for in time the outer shell itself began to wear away, and the 

 cup became sufficiently light to be moved by the wind, and many had been lifted up 

 and smashed to pieces. 



One other testimony to the power of the plateau wind as a denuding agent was 

 well shown, at a height of 6000 feet, on the Terracotta Mountain, where the talus 

 became sufficiently thin to allow the Beacon Sandstone to be seen in situ. One of 

 the beds of sandstone was a fine-grained white rock, and this stood out in ledges 

 18 or 20 inches wide, and the underside of these ledges had been weathered into 

 a series of thin, roughly hexagonal columns from 1 to 9 inches long, and from J to 

 i an inch thick. 



Where these columns hung close together their original structure seemed, as 

 before mentioned, to be hexagonal, and it appears probable that the weathering has 

 been assisted in producing this particular result by some secondary structure due 

 to alteration and secondary crystallisation in the rock itself When the columns 

 hung farther apart, owing to the gaps made by the entire removal of some, they 

 had lost all definite shape, and resembled nothing so much as stone icicles or 

 stalactites. 



4. Due to Chemical Action. Chemical weathering plays a very subordinate part 

 at Cape Royds, although the water around some of the large boulders in summer 

 tasted quite strongly of magnesium. This type of weathering is much more 

 developed in the western mountains, and it seems probable from the small part 

 it plays here that the kenyte is not so easily decomposed as other rocks of the 

 region. One phenomenon, which would lead one to an erroneous view of the 

 importance of this agency, is the presence of an efflorescence of salts all over Cape 

 Royds, but these salts are mostly sodium sulphate and chloride, and their common 

 occurrence, when the removal of snow-drifts has caused concentration of the salts 

 contained in the drift, seems to point to their having been brought originally as 

 sodium chloride from the surface of the sea ice, so that the main chemical change 

 that seems to be taking place in the rock material is the filching of the sulphates 

 and their replacement by chlorides. The minerals most probably taking part in this 



