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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. Zoölogy, Harvard College. Vol. 16, No. 12.(Geological Series, Vol. 2).

Contributions to our knowledge of the mineral and chemical composition as well as the relationships of the igneous rocks of particular regions, however fragmentary, are of the greatest importance; especially 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 Colorado and Wyoming along the fortieth parallel of latitude, by the geologists under Mr. Clarence King, constitutes the one great systematic 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 immediately east of it. This difference consists mainly in the greater abundance 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

  1. ↑ J. P. Iddings: The Origin of Igneous Rocks.Bull. Phil. Soc. Washington, Vol. 12, pp. 138, 139, 184.

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