56 DISCOVERY REPORTS 



Dr Mackintosh's collection have been sliced, and they are found to be singularly uniform in com- 

 position. They are made up of irregular, angular, and highly vesicular lapilli and scoria, the fragments 

 usually var^'ing in size between a hazel-nut and a walnut. The fragments consist of glassy forms of 

 both the basic and acid andesitic types, the black opaque slaggy form and the clear glassy form being 

 about equally abundant. The glassy fragments are frequently of a bright yellow colour, but some are 

 brown and a few others of a greenish tint. Many of these fragments have a narrow border of the 

 black opaque variety, suggesting that the separation of magnetite dust in the glass which gives rise 

 to the opacity may be due to a reheating or annealing process. There is little or no matrix of finer 

 material between the fragments, and they appear to be welded together along their contacts. This 

 material therefore might be better classed as agglutinate'^ than as agglomerate. 



Tridymite (and cristobalite?) occurs abundantly in these fragmental rocks, not only lining the 

 vesicles of the glassy fragments, but also as an edging around the individual fragments. This suggests 

 that, in these rocks at any rate, the tridymite is of deuteric crystallization. It has been formed shortly 

 after the consolidation of the fragmental material, and is no doubt due to late emanations derived 

 from the parent magma. 



SNOW ISLAND 



This is a small island west of Livingston Island, and west-north-west of Deception Island. It is 

 geologically unknown, and no description and no record of any landing is known to me. Four specimens 

 of rocks from Snow Island, however, were found in the first set of material sent to me by the Discovery 

 Committee, with no record when and by whom collected. Three of the rocks appear to have been 

 collected in situ from actual exposures, but the fourth is a pebble from a raised beach at 50 ft. above 

 present sea-level on the eastern coast of the island. 



Of the three specimens collected />/ situ on the eastern side of the island one is a quartz-pyroxene- 

 diorite or feldspathic quartz-gabbro of a type identical with other occurrences in the South Shetland 

 Islands and the Palmer Archipelago; the second is an oligoclase-andesite breccia with a tuffaceous 

 matrix containing a good deal of quartz. The third is a quartz-felsite or rhyolite with a scanty crypto- 

 crystalline matrix. The pebble from the raised beach is quartz-augite-microdiorite, identical with the 

 quartz-pyroxene-diorite above mentioned except that it contains patches of fine-grained ground-mass. 



Even from this scanty material, therefore, the indications are clear that the constitution of Snow 

 Island is the same as that of the other islands of the South Shetlands group, and that rocks of the 

 older igneous series are here represented. 



DREDGINGS FROM BRANSFIELD STRAIT 

 A few score of stones dredged from two stations in Bransfield Strait were included in the first 

 collection of rocks received from the Discovery Committee. These came from St. 175, about 25 miles 

 south-east of Deception Island, and St. 177, about 27 miles south-west of Deception Island, and were 

 dredged from depths of 200 and 1080 m. respectively. The stones were probably dropped from the 

 ice which formerly occupied Bransfield Strait, and which probably moved from the west and south- 

 west. Some of the material may have been carried by icebergs breaking away from glaciers on the 

 South Shetlands and the Graham Land coast. The specimens range in size from blocks 6 in. across 

 to \ in. pebbles. Most of them are angular and facetted, with corners and edges roughly rounded 

 oflF; only a few appeared to be well-rounded, apparently water- worn pebbles. 



As was to be expected, the great majority of the seventy-nine stones sliced consist of the older 

 series of andesites, dacites, rhyolites, agglomerates and volcanic breccias, which appear to constitute 



^ G. W. Tyrrell, Volcanoes {Ylome. University Library), 193 1, p. 66. 



