MUSEUM OF COMPARATIVE ZOOLOGY. 2^^j3 



lacking ; they arc in some cases isotropic, have often a polygonal shape, 

 and the acmite needles, which are abundant in the rock, arrange them- 

 selves parallel to their sides when in proximity ; they also gelatinize 

 strongly with acid, and thus resemble the small irregular sodalite ci'ys- 

 tals of the Montana rocks. The feldspar phenocrysts of the Siebenge- 

 birge rock have the triclinic twinning described above, and thus the two 

 rocks are nearly identical, at least for the American variety with little 

 nepheline. 



The Sijenitic type resembles in appearance and structure the elreolite- 

 syenite ("gray granite") from Arkansas, described by J. F, Williams, 

 in chemical and mineralogical character. It is closely allied to the Mon- 

 tana rock, excepting that it has more nepheline, and that the feldspar 

 was referred by Williams to the microcline-microperthite of Brcigger (a 

 microscopic interlamination of microcline and albite), while the rea- 

 sons are given above for considering the feldspar of the Montana rock 

 a microscopically homogeneous triclinic soda-potash feldspar. Lind- 

 gren {loc. cit.) has described as '' augite-trachytes " rocks from the High- 

 Avood Mountains closely resembling these. 



The previous descriptions illustrate the dependence of rock structure 

 on physical conditions of cooling, which is so striking a feature of the 

 eruptive I'ocks of this range, the syenitic or trachytic character of the 

 I'ock depending on the variation in the thickness of the rock mass. 



Camisridge, Mass., January, 1893. 



