ROCK SPECIMENS 



193 



these dacitic lavas in the West Antarctic region is a quartz-diorite (Table I, B) from a 



dredged block. The most comparable Andean lava is a dacite from Colombia (Table I, C). 



The average dacite (Table I, D) compares closely with the rock under investigation, 



but is richer in potash relatively to soda, and is somewhat less siliceous. 



The normative mineral compositions of these rocks, computed under the rules of 



the American Quantitative Classification, are set out in Table II. The Thule Island 



dacite is there shown to compare remarkably well with the average dacite (Table II, D), 



but is somewhat richer in quartz and poorer in orthoclase, features which are also 



shown by the Colombian dacite (Table II, C). The assignment of the Thule Island lava 



to the dacite group is therefore well founded ; but since the quartz is mostly occult in 



the cryptocrystalline ground-mass, Lacroix's term dacttoid,^ applied to rocks of this 



chemical character in which quartz is not mineralogically expressed, is more appropriate 



than dacite. On the evidence of the minerals actually seen in thin section, and without a 



corrective chemical analysis, this rock would probably have been called oligoclase- 



andesite. 



Table II 



Norms of Analyses in Table I. 



The norms are arranged in the same order as the analyses in Table I. The symbols of these analyses 

 in the American Quantitative Classification are given below: 



(3) 4 (2) 3 '4 



(II) 4 "2 4" 



4 "3 4 



"4 2" 4 



4 (2)3 4 



I. 

 A. 

 B. 

 C. 

 D. 



The andesites are hypersthene-augite-andesites usually containing a little olivine. In 

 some specimens the olivine becomes a fairly prominent constituent, but without lessen- 

 ing the andesitic character of the rocks, and the term oHvine-a?idesite may then be 

 employed. Olivine-andesites, often called andesitic basalt, basaltic andesite, or simply 

 basalt, are common in many andesitic lava fields. They are, however, easily distinguish- 

 able on chemical and mineralogical grounds from true basalts. 



1 Comptes Rendus, Paris, CLXViii, p. 297 (1919). 



