192 



DISCOVERY REPORTS 



constituents of these rocks. Quartz does not appear, although the chemical analysis 

 (see Table II, i) discloses no less than 30 per cent of free silica. 



Two of the slides show included fragments of a coarse-grained olivine-andesite 

 consisting of plagioclase laths, elongated crystals of augite studded with granules of 

 magnetite, and sporadic olivine, in a glassy ground-mass which has been partially 

 devitrified with the production of ill-defined feldspathic material and quartz. The 

 angularity of these xenoliths, and the fact that they have obviously chilled a sur- 

 rounding zone of lava with the production of an opaque envelope of iron ores, indicate 

 that they are fragments picked up by the lava as it flowed over the surface, and not 

 cognate xenoliths brought up from great depths. 



Table I 

 Analyses of Dacites, etc. 



1. Dacite (Dacitoid), Thule Island, South Sandwich Islands. Anal. F. Herdsman, A.R.S.M. 



A. Hyalodacite ("Trachyte"), Deception Island, South Shetland Islands. Quoted from H. S. Washington, 



"Chemical Analyses of Igneous Rocks", Prof. Paper 99, U.S. Geol. Siirv. pp. 242-3 (1917). Also see 

 G. W. Tyrrell, Trans. Roy. Soc. Edin. Llii, pt i, p. 71 (1921). 



B. Quartz-diorite, dredged block from sounding in lat. 70° S, long. 81° W (Paris), West Antarctica. Quoted 



from H. S. Washington, op. cit. pp. 266-7. 



C. Dacite, Guaitara Slope, Loma de Ales, Colombia. Quoted from J. P. Iddings, Igneous Rocks, 11, p. 496 



(1913)- 



D. Dacite, mean of thirty analyses. Quoted from G. W. Tyrrell, Principles of Petrology, p. 124 (1926). 



The chemical analysis of a composite sample of three specimens of dacite lava from 

 the 50 ft. cHff is given in Table I, i. It is there compared with an analysis of the 

 hyalodacite of Deception Island in the South Shetland group, which appears to be its 

 closest affinity in the West Antarctic region (Table I, A). The analyses are closely 

 comparable, although the Deception Island rock has higher total alkalies, and a higher 

 ratio of soda to potash, than the Thule Island rock. The closest plutonic analogue to 



