554 



MR G. W. TYRRELL. 



there compared with the type shonkinite of Square Butte, Montana, and with a 

 shonkinitic syenite from Middle Peak in the same district. 



Table V. 





1. 



2. 



3. 



Alkali-felspar * . 



610 



20 



532 



Nepheline 









5 





Sodalite . 







2-0 



1 





Biotite 









8 



4-0 



Titanaugite 







14-1 



46 



32-3 



Barkevikite 







13-4 



. . . 





Olivine 









10 



1-7 



Iron-ores t 







7-5 



6 



7-9 



Sphene 







■6 







Apatite 







1-4 



4 



•9 



1. Shonkinite, Ochilesa, Benguella. 



2. Shonkinite, Square Butte, Montana. Quoted from Pirsson, U.S. Geol. Surv., 



Bull. 237, 1905, p. 104. 



3. Shonkinitic syenite, Middle Peak, High wood Mountains, Montana ; ibid., p. 93. 



• 

 The Square Butte rock is much richer in the mafic constituents than that of 

 Ochilesa, which resembles the Middle Peak shonkinitic syenite much more closely. 

 As is shown in the discussion of the chemical analyses, the type shonkinite of Square 

 Butte tends now to become an extreme type as compared with the average of the 

 rocks which have been grouped as shonkinites ; and the Ochilesa rock is very close 

 to the average shonkinite. It differs from all previously described shonkinites in 

 containing abundant hornblende. It compares well with the Montana types in con- 

 taining a little sodalite, and in being associated with sodalite-syenites. 



The chemical composition of the Ochilesa shonkinite has been kindly determined 

 for me by A. Scott, M.A., B.Sc. The figures are set out in Table VI, 1 ; and are 

 there compared with the type shonkinite of Square Butte, and with the average 

 shonkinite. The composition of the latter has been derived from the average of 

 twenty-one analyses of shonkinitic rocks found in Iddings' Igneous Rocks, vol. ii 

 (1913), and in Rosenbusch's Gesteinlehre, 3rd ed., 1910. In the collection is included 

 the shonkinites of Montana, Arkansas, Celebes, and the Katzenbuckel, Odenwald ; 

 also the malignites (nepheline-pyroxene-garnet-shonkinites) of Ontario and British 

 Columbia ;■ and the marosites (biotite-augite-shonkinites) of Celebes. The range of 

 rocks covered by this average corresponds to that covered by Rosenbusch's defini- 

 tion and descriptions, and consequently includes nepheline-bearing types. Certain 

 " microshonkinites " described by Lacroix from the Los Islands, West Africa, are 

 excluded, since they are far too rich in nepheline to come under the heading of 



shonkinite. 



* Mostly soda-orthoclase. t Magnetite and ihnenite. 



