366 DISCOVERY REPORTS 



expansion will slowly become worn down and eventually disappear through exposure to 

 freeze and thaw action and the undermining of the ice on either side ; and with the 

 gradual disappearance of their constricting walls adjoining glaciers will coalesce to form 

 fringing ice platforms of increasingly greater frontage. Such a development would 

 already appear to have taken place in some measure on the east coast of Coronation 

 Island, for there, projecting through relatively broad stretches of ice, various nunataks 

 appear, isolated rock pinnacles that represent perhaps the remnants of former dividing 

 ridges (see Plate XXI, fig. i). 



Note on rock joints. As far as we are at present aware the most commonly occurring 

 rocks are (i) sediments of several kinds, principally greywackes, found mainly in the 

 eastern portion of the group on Laurie, Saddle and Fredriksen Islands, and (2) highly 

 metamorphic gneissose rocks confined apparently to the western portion, to Signy 

 Island and certain parts of Coronation Island. The Inaccessibles in the extreme west 

 appear to be composed of greywacke. According to Pirie^ the typical rock of Laurie 

 Island is a fine-grained, almost homogeneous greywacke of a blue-grey or greenish 

 grey colour traversed irregularly by thin quartz or calcite veins. Bedding on the whole 

 is indistinct, the rocks being massive in character and much traversed by cracks and 

 faults which give them a shattered appearance. In certain cliff faces the rock shows well- 

 marked jointing, often very difficult to distinguish from bedding planes. 



Shattering and jointing such as Pirie describes were seen by us at most of the localities 

 we visited from Wilton Bay westwards to the Inaccessibles. They are evidently of wide 

 occurrence, not only on Laurie Island but throughout the group as a whole, and would 

 appear to be largely responsible for certain peculiarities of cliff formation on the coasts. 

 Along the eastern side of Wilton Bay for example there are vertical cliff^s and steep head- 

 lands rising to 200 or 300 feet above the sea. The rock is a greywacke much traversed by 

 cracks and joints, the latter in most instances vertical or nearly so, and varying from a 

 few inches to several feet in width. Near the water-level, where the vertical fissures are 

 most conspicuous, wave and possibly ice action have evidently played a considerable part 

 in excavating and widening them, for through some it is possible to take a ship's boat. 

 In other parts of Laurie Island remarkable formations such as the great vertical spires or 

 teeth of rock at Cape Hartree (Plate XXIII, fig. 3) and similar jagged, although less im- 

 posing, structures on the northern coast, while perhaps reflecting in a measure the steep 

 angle at which these sedimentary beds are said to be inclined, may well be associated 

 with vertical jointing, such as has been described, in conjunction with freeze and thaw 

 action in the joints. At Cape Hansen on the south coast of Coronation Island, certain rock 

 faces have clearly been produced by vertical jointing with subsequent flaking off of con- 

 siderable portions of the rock. The rock is gneissose and traversed profusely by parallel 

 vertical fissures up to four or five inches in width. Some are open, others occupied by 

 bands of secondary quartz. The buttress behind the cape rises for upwards of 400 feet by 

 a series of stepped platforms, each backed by a more or less smooth and vertical rock face. 



1 Pirie, J. H. Harvey, 1905, On the Graptolite-bearing Rocks of the South Orkneys, Proc. Roy. Soc. 

 Edin., XXV, part vi, pp. 463-8. 



