550 



MR G. W. TYRRELL. 



hand, the average dacite is poorer in silica, and has a decided predominance of soda 

 over potash. In most respects, therefore, the Quingenge rhyolite is intermediate 

 between average rhyolite and dacite. The analysis of the type dellenite from Sweden 

 (Table III, 4) is very similar to that of the Quingenge rock. It is, however, richer 

 in potash and poorer in lime. The Swedish rock contains hypersthene, and was 

 originally called hypersthene-andesite ; but the analysis shows that this identifica- 

 tion was completely astray. It is worth while comparing the norms of these rocks 

 (Table IV). 



Table IV. 





1 



2. 



Quartz .... 



Orthoclase 



Albite .... 



Anorthite 



Femic minerals, water, etc. 



25-4 

 20-6 

 33-0 

 13-6 



7-4 



25-6 



26-1 



30-9 



8-3 



91 



1. Rhyolite (dellenite), Quingenge, Benguella. 



2. Dellenite, Dellen, Helsingland, Sweden. 



The norms correspond very closely, especially in regard to the salic constituents, 

 to the modes that would have been developed had the rocks been holocrystalline. 

 The dellenite is shown to be somewhat richer in orthoclase and poorer in oligoclase 

 than the Quingenge rock, but the figures are sufficiently close to justify the identi- 

 fication of the latter as dellenite. I have not been able to find records of ancient 

 African rhyolites similar to those of Benguella, but some of the Tertiary rhyolites of 

 Algeria approach them closely in chemical and mineral composition. In the American 

 Quantitative Classification the dellenite of Quingenge falls into the subrang lassenose, 

 but is transitional to toscanose. The type dellenite of Sweden falls into toscanose. 



VI. The Alkaline Series. 



An interesting series of alkaline rocks occurs in Benguella in the district of 

 Ochilesa, the extreme north-eastern point reached by Professor Gregory. At 

 Chieuca these rocks are found within a volcanic vent, associated with tuffs and 

 igneous breccias composed of the same materials ; but there are also intrusive masses 

 within the district which are not in actual connection with the vent. 



The principal alkaline types found in the collection are nepheline-sodalite-syenite 



and other types of syenite, shonkinite, solvsbergite, and ouachitite, which are treated 



in order below. 



1. Syenites. 



A. Nepheline-sodalite-syenite. — This type occurs as an intrusion at Ochilesa (209). 

 It is a thoroughly leucocratic or perfelsic, coarse-grained rock, consisting n ainly of 



