416 WEED AND PIRSSON — HIGHWOOD MOUNTAINS OF MONTANA. 



and thereby related to the syenite family. It may be with or without 

 olivine, and accessory nepheline, sodalite, etcetera, may be present in 

 small quantities. The Square butte rock is thus olivine-shonkinite, with 

 these accessory minerals. 



The fine grained dense porphyritic forms which bear the same relation 

 to shonkinite that trachyte does to syenite are dark to black heavy basalts. 

 They are, in fact, orthoclase basalts, a type which although so far as we 

 know has not yet been described from European localities, is by no means 

 rare in western America. Besides its occurrence in the Highwoods, and 

 also in other localities in Montana alluded to by Lindgren,* its presence 

 in the Absaroka range and Yellowstone National Park has been men- 

 tioned by Iddings.t Somewhat similar rocks have been also mentioned 

 by Zirkel,J who does not, however, discuss this type of basalts in the 

 recent edition of his great work on petrography, so far as we have been 

 able to discover in the absence of complete indexing. 



White Rock or Sodalite-syenite. — The petrograph}^ of the light colored 

 inner core of the denuded laccolite has been so completely investigated 

 by Lindgren and Melville § that a further examination enables us to add 

 but very little to their comprehensive description. The rock is shown to 

 be a sodalite-syenite, and for pur230ses of convenience we briefly sum- 

 marize their results, referring to the original paper for fuller information. 



Megascopically the rock when very fresh is nearly pure white, often 

 with a brownish to pinkish tinge, consisting mainly of feldspar, which 

 often reaches 5 millimeters in diameter. Through this are scattered 

 slender, glittering black hornblende prisms which attain at times the same 

 length. It is scarcely sufficient in amount to detract at a distance from 

 the general whiteness of the rock. Small grains of a salmon to brown 

 colored sodalite are also present. The rock is thus rather coarsely gran- 

 ular, and in fact of the same size grain as the shonkinite, with which it 

 is so intimately connected. 



The microscope shows the following minerals present in the order of 

 their formation : Apatite, hornblende, orthoclase (with some albite), sodal- 

 ite and analcite. The hornblende is in slender prisms bounded by m. 

 110 and b, 010, terminations wanting, frequently twinned on a (100). It 

 is strongly pleochroic C and t), deep brown a, yellowish broAvn and ab- 

 sorption very great t) = C > a. An outer mantle often shows a greenish 

 color (from change into the arfvedsonite molecule? — L. V. P.). Angle 

 c A C = 13 degrees ; is idiomorphic against the feldspathic constituents. 



* Loe. eit., p. 50; also, Am. Jour. Sci , vol. 45, 1893, p. 289. 

 t Bull. Phil. Soc. Washington, vol. 12, 1892, p. 169. 

 t Mie. Petrog. Fortieth Par., 1876, p. 225. 

 § Loc. cit. 



