STRUCTURE OF THE SQUARE BUTTE DARK KOCK. 



415 



that in this dark rock of Square biitte we liave a granular, plutonic rock, 

 composed essentially of aiujlte and orthoclase, with smaller amounts of 

 olivine and iron ore and witli accessory apatite, sodalite, nepheline, 

 etcetera. In its chemical composition it stands very close to certain 

 vogesites and minettes — basic rocks of the syenitic group. It differs 

 from them essentially, first, in its mineral composition and, second, in 

 its structure. For a rock of its character there seems to be no position 

 in any of our i)re3ent schemes of classitication. It would be manifestly 

 improper to term such a rock an augite-syenite, as its chemical com[)o- 

 sition removes it very tar from 

 syenites. It bears indeed such a 

 relation to augite-syenite as vog- 

 esite does to hornblende-syenite ; 

 that minette or, perhajxs better, 

 the I)url)achite of Sauer-'^ does to 

 mica-syenite. 



It stands generally related in- 

 deed to rocks of the basic class — 

 low in SiO,, high in ^[gO, ( aO 

 and FeO, and tiiereby related to 

 rocks of the lamprophyre family. 

 Moreover, this type is found in 

 the High woods not only in the 

 outer mantle of S(piare butte, al- 

 though constituting there an im- 

 mense mass, but at many other 

 points forming great intrusive 

 stocks. As briefly noted by Lind- 

 gren,t the variability of tiie augite and orthoclase in the Highwood rocks 

 is ver}' great. As in the gabbro family we have every range from anor- 

 thosite at one (^ul to i)eridotites at the other, with the gabl)ros standing 

 in an internjediate })osition,S() in the llighwoods variation extends from 

 syenites i)ractically devoid of ferro magnesian minerals to those in whicli 

 augite becomes the chief constituent, though the basic extreme entirely 

 devoid of feldspar has not been observed by us. 



Name Shonkinite. — For this type of rock, then, we i)ropose the name 

 of shonkinite, from shonkin, the Indian name of the Highwood range, by 

 whicli name, indeed, it is still called by many, and shonkinite we define 

 as a granular plutonic rock consisting of essential augite and orthoclase, 



Fkjuue S.-Micro-drnwitif/ of Shonkinite multiplied 14 

 Dia7iieters. 

 A = uugite ; — olivine ; £ = biotite ; Or = or- 

 tliochvse ; An = anoitlioclase ; A — apatite. Actual 

 field, 4 mm. 



* Mitt. d. Bad. geol. LutideHani^talt, ii Hd., p. 217. 



tProc. California Acad.Scl., .-^er. 2, vol. iii, p. 47. Tentli Census, vol. xv, p. 



