SOUTH SHETLAND ISLANDS 51 



composed of lathy plagioclase (about AbiAiii), pale brown augite, and iron ores. Both feldspar and 

 augite occasionally attain micro-porphyritic dimensions. The abundant fresh olivine, however, forms 

 large phenocrysts. 



A 'common type' along the shore is a basalt with numerous small feldspar phenocrysts, and less 

 numerous olivine and augite crystals, embedded in a ground-mass of intersertal texture. This recalls 

 the Dunsapie type of the Scottish Carboniferous, as was also remarked by Dr Thomas. Another type 

 which appears to be abundant in this locality is one with an intergranular ground-mass exceedingly 

 rich in augite. Dr Thomas described rocks of this type but, unlike our specimen, his material contained 

 much olivine. Some of these augite-basalts, as they might be called, carry numerous little prisms of 

 l(iw double refraction and straight extinction which are identified as enstatite, in the ground-mass 

 along with the monoclinic pyroxene. This is an enstatite-basalt. Dr Thomas described a similar rock 

 as hypersthene-basalt. 



While most of the specimens collected here are basalts, one is an augite-andesite of the common 

 type belonging to the younger lava series. It is accompanied by an andesitic agglomerate. Beach 

 pebbles collected from Coppermine Cove consist of tonalite and granite-aplite. 



LIVINGSTON ISLAND 



Livingston Island is the second largest of the South Shetland group, but very little is known of 

 its geology. Mr Ferguson collected an olivine-basalt from a fine columnar exposure forming a small 

 island off the coast in M^Farlane Strait (Edinburgh Hill), and noted tuff's in the vicinity which, beside 

 basalt, contained fragments of quartz-diorite and black mudstone [op. cit. p. 43 and pi. i, fig. i). 



Desolation Island. Dr Mackintosh collected a few specimens from Desolation island which lies 

 off the northern coast of Livingston Island. He gives no geological details except that the island is 

 mainly composed of a columnar igneous rock. It is noteworthy that on the Discovery Chart (Discovery 

 Reports, vol. vi, 1932, Chart 6) Desolation Island is represented in the shape of an irregular broken 

 ring, suggesting that it may be a breached crater flooded by the sea ; but this resemblance may, of 

 course, be quite accidental. 



Two of the specimens were collected in situ from columnar outcrops. Both are very fresh and coarse- 

 grained hypersthene-basalts of an unusual type. The major part of both rocks consists of a coarse 

 intergranular admixture of laths of labradorite (Auen-An^o) with granules of pale green augite, prisms 

 of enstatite-hypersthene with faint pleochroism, and iron ores. The feldspar and augite occasionally 

 form somewhat larger micro-porphyritic crystals, but the rock is not conspicuously porphyritic. 

 Both kinds of pyroxene, moreover, tend to build small aggregations or clots, which stand out as a 

 glomero-porphyritic texture. Olivine occurs only sparingly as small pseudomorphs in brownish 

 serpentine. A small amount of dark brown glass fills up interstices in the ground-mass. 



A basalt with orthorhombic pyroxene in the ground-mass was described by Thomas from Roberts 

 Island ((2), p. 86). OUvine did not occur in this rock, and the augite occasionally formed glomero- 

 porphyritic aggregates. A closely comparable rock from the same locality has been described in this 

 paper (p. 51). These rocks are no doubt closely related to the basic hypersthene-augite-andesites 

 above described, which are so common in the South Shetland Islands. In these rocks, however, the 

 hypersthene is porphyritic and does not occur in the ground-mass. Barth and Holmsen have given 

 an interesting discussion of the petrographical problem involved in the presence of hypersthene in 

 these rocks ((4), pp. 14^17)- 



Numerous pebbles from the beaches and fragments from the screes of Desolation Island were 

 collected. These include tonalite and a sericitized and chloritized diorite, silicified andesitic breccia, 

 and a series of acidic volcanic rocks including a fluxional rhyolite or dacite with augite, a rhyolitic 



