A CONTRIBUTION TO THE PETROGRAPHY OF BENGUELLA. 



549 



Some of the specimens from Lepi are compacted rhyolite-tuffs (225, E. of Lepi). 

 These contain numerous fragments of quartz, oligoclase, orthoclase, and microcline, 

 along with numerous angular fragments of microcrystalline quartz-porphyry, in a 

 dense, cryptocrystalline groundmass. 



A rock generally similar to the rhyolites described above, from W. of Cruz's, 

 Quingenge (234), is much poorer in phenocrysts, and is devoid of phenocrystic 

 quartz. 



A chemical analysis of one of these rocks (258, Quingenge) has been made for me 

 by A. Scott, M.A., B.Sc, with the result set out in Table III, 1. 



Table III. 





1. 



2. 



3. 



4. 



Si0 2 . •. 



68-98 



72-60 



6691 



69-48 



Ti0 2 . . 



•23 



•30 



•33 





A1 2 3 . 



15-24 



13-88 



16-62 



13-88 



Fe 2 3 . 



2-83 



1-43 



2-44 



2-67 



FeO 



•43 



•82 



1 33 



1-53 



MnO 



•05 



•12 



04 



•15 



MgO . . 



1-08 



•38 



1-22 



•71 



CaO 



3-01 



1-32 



3-27 



2 39 



(Ba, Sr)0 



tr. 









Na 2 . 



3-88 



3-54 



413 



374 



K 2 . 



353 



403 



250 



4-44 



H 2 0+ . 

 H 2 - . 



•53 ) 

 •39 ) 



1-52 



113 



1-19 



PA . 



•09 



•06 



•08 





C0 2 . . 



nt. fd. 









100-27 



100-00 



100-00 



100-18 



1. Rhyolite (dellenite), Toscanose-Lassenose (T, 4, 2', (3) 4), Quingenge, Benguella 



(258). Anal., A. Scott. 



2. Average rhyolite, quoted from Daly, Igneous Rocks and their Origin, 1914, p. 19. 



3. Average dacite, ibid., p. 25. 



4. Dellenite, Toscanose (V, '4, 2, 3), Dellen, Helsingland, Sweden. Quoted from 



Iddings, Igneous Rocks, vol. ii, 1913, p. 128. 



Consideration of this analysis, combined with what is known of the mineral composi- 

 tion, shows that the Quingenge rocks are the lavaform equivalents of the grano-diorite 

 rather than of the granite family, and belong therefore to the group called dellenite 

 by Brogger.* The toscanites of Italy are chemically, but not mineralogically, similar 

 to the Benguella type. The averages of sixty -four analyses of rhyolite, and of thirty 

 dacites, as calculated by Daly,! are given for comparison in Table III. The average 

 rhyolite is richer in silica, poorer in lime, and has a slightly greater content of 

 alkalies in which potash is predominant, than the Quingenge rock. On the other 



* See Iddings, Igneous Rocks, vol. ii, 1913, p. 104. t Igneous Rocks and their Origin, 1914, pp. 19, 25. 



