36 Pirsson — Petrographic Province of Central Montana. 



Norway, and our knowledge of this region we owe for the 

 greatest part to the keen perception and untiring labors of 

 Brogger, who has given the results of his work in that fine 

 series of monographs which have become classics in the litera- 

 ture of petrography. 



The fact that the outlying mountain groups east of the main 

 chain of the northern Kocky Mountains are composed of rocks 

 of a special character rich in alkalies, was pointed out by 

 Iddings- in the work already referred to, although at that 

 time little was known about them. Since then investigations 

 and studies in the field and in the laboratory by a number of 

 workers have thrown a flood of light upon this region. In 

 the Black Hills of North Dakota the work of Caswell,f Jaggar,^ 

 Irving§ and the writer) has shown a prevalence of types rich 

 in alkalies with soda dominating the potash. 



In Montana, the most southern of the eastern outlying 

 groups fronting the great plains, is the Crazy Mountains, some 

 of whose interesting rocks of alkalic types are known through 

 the researches of Wolff. T North of this come the various 

 groups studied by Mi*. Weed and the writer ; the Castle Moun- 

 tains •** the Little Belt Mountains ;f+ the Judith Mountains ;$:): 

 the Highwood Mountains ;§§ the Bearpaw Mountains ;|||| the 

 Little Rocky Mountains! "1" and lastly, on the border line between 

 Canada and the LTnited States, the Sweet Grass Hills,**"* the last 

 of the outliers. While some of these have been rather thor- 

 oughly investigated, there yet remains much to be done. The 

 few types that have been described from the Crazy Mountains 

 by Wolff , and its mapfff showing the vast complexity of the 



*Op. eit., p. 31. 



f Microscopic Petrography of the Black Hills, 1876. U. S. Geog. and 

 Geol. Surv., Rocky Mts. region. J. W. Powell in charge. Rep. on the Black 

 Hills of Dakota, pp. 469-527, Washington, 1880. 



X Laccoliths of the Black Hills, 21st Ann. Rep. U. S. Geol. Surv., Pt. iii, pp 

 163-290, 1901. 



§ Geology of the northern Black Hills. Ann. N. Y. Acad. Sci., vol. xii 

 No. 9, pp. 187-340, 1899. 



|| Phonolite Rocks from the Black Hills. This Journal, 3d Ser., vol xlvii 

 pp. 341-346, 1894. 



IT Bull. Geol. Soc. Amer., vol. iii. pp. 445-452, 1892. Bull. Harv. Mus 

 Comp. Zool., vol. xvi, pp. 227-233, 1893. 



** Bull. No. 139, U. S. Geol. Survey, 1896. 



ft 20th Ann. Rept. U. S. Geol. Surv., 1900, Pt. iii. p. 562. This Journal 

 3d Ser., vol. 1, pp. 467-479, 1895. 



ftlSth Ann. Rept. U. S. Geol. Surv., 1898, Pt. iii, p. 437-616. 



8§Bull. 237 U. S. Geol. Surv., 1905. Bull. Geol. Soc. Amer., vol. vi, pp 

 389-422, 1895. This Journal, vol. ii, pp. 315-323, 1896. 



HI This Journal, 4th Ser., vol. i, pp. 283-301, 351-362, and vol. ii, pp 

 136-148, 188-189, 1896. 



1TTT Jour, of Geol., vol. iv, pp. 339-428, 1896. 



***This Journal, 3d Ser., vol. 1, pp. 309-313, 1895. 



fff Little Belt Mountains Folio, Montana. U. S. Geol Surv., Geol. Atlas of 

 U. S., No. 56, 1899. 



