MUSEUM OF COMPARATIVE ZOOLOGY. Ano 
lacking ; they are 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 crys- 
tals of the Montana rocks. The feldspar phenocrysts of the Siebenge- 
birge rock have thertriclinic twinning described above, and thus the two 
rocks are nearly identical, at least for the American variety with little 
nepheline. 
The Syenitic type resembles in appearance and structure the eleolite- 
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 Brégger (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 (/oc. cit.) has described as “ augite-trachytes ” rocks from the High- 
wood 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 rocks of this range, the syenitic or trachytic character of the 
rock depending on the variation in the thickness of the rock mass. 
CAMBRIDGE, Mass., January, 1893. 
