Reviews. 



Eruptive Rocks from Montana. By Waldemar Lindgren. Proc. Cal. 

 Acad. Sci. Ser. 2, Vol. 3. 1890. 



A Sodalite- Syenite and other Rocks from Montana. By W. Lindgren, 

 with analyses by W. H. Melville. Am. Jour. Sci. Vol. 45. 

 April 1893. 



Acmite- Trachyte from the Crazy Mountains, Montana. By J. E. Wolff 

 and R. S. Tarr, Bull. Mus. Camp. Zoology, Harvard College. 

 Vol. 16, No. 12. (Geological Series, Vol. 2). 



Contributions to our knowledge of the mineral and chemical com- 

 position as well as the relationships of the igneous rocks of particular 

 regions, however fragmentary, are of the greatest importance ; espe- 

 cially when they relate to the vast areas of North America which remain 

 almost unknown to the petrologist. The exploration of the great belt 

 of country, one hundred miles wide, extending from California to Colo- 

 rado and Wyoming along the fortieth parallel of latitude, by the 

 geologists under Mr. Clarence King, constitutes the one great system- 

 atic study of the volcanic rocks of any considerable area on this 

 continent. Less extensive investigations of smaller areas, isolated 

 from one another and often separated by long distances, have been 

 made from time to time, and to some extent have been published. 

 But a large part of the work already done has not yet been printed. 

 The facts so far brought to light show that the rocks of the Great 

 Basin and the Pacific coast differ as a whole from those occurring in 

 the eastern portion of the Rocky mountains and the region immedi- 

 ately east of it. This difference consists mainly in the greater abund- 

 ance of the alkali-bearing rock-making minerals in the rocks of the 

 latter region, caused by the relatively higher percentage of sodium or 

 potassium in the magmas from which they have been derived. 1 



The recent papers by Mr. Lindgren and by Messrs. Wolff and Tarr 

 illustrate this characteristic of the volcanic rocks of Montana along the 

 frontal ranges of the Rocky mountains. All of the rocks described 

 occur as intrusive bodies ; laccolites, sheets, dikes or necks. They 



'J. P. Iddings : The Origin of Igneous Rocks. Bull. Phil. Soc. Washington, Vol. 

 12, pp. 138, 139, 184. 



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