ERUPTIVE ROCKS FROM MONTANA. 



BY W, LINDGREN, U.S. GEOI.. SURVEY. 



During the summer of 1883, while engaged in the geo- 

 logical survey of the lands adjacent to the Northern 

 Pacific Railroad (The Northern Transcontinental Survey) 

 under the direction of Mr. R. Pumpelly, I visited many 

 parts of the Rocky Mountains in Montana, as a member 

 of the reconnoitering party of Mr. W. M. Davis. I chiefly 

 directed my observations to the occurrence and character 

 of the eruptive rocks, and during the following winter ex- 

 amined a number of them microscopicallv. The results 

 of this examination were subsequently published by Mr. 

 Pumpelly in Vol. xv, loth Census, p. 719, together with 

 other investigations of the Northern Transcontinental 

 Survev. 



Hoping to secure new points of view and perhaps cor- 

 rections in the old determinations I have recently reviewed 

 the evidence and the conclusions to which I had arrived, 

 and the results of this review are embodied in this paper. 

 Occasional changes in the nomenclature of the rocks de- 

 scribed have been made and especially the analcite basalts 

 subjected to a detailed re-examination. 



INTRODUCTION. 



The effusive volcanic rocks of later tertiary and recent 

 date are conspicuouslv absent in the Belt Mountains or 

 Front Ranges as well as in the Main Range of the Rocky 

 Mountains in Montana. Eruptive rocks are, however, not 

 wanting; indeed thev form a prominent part of the geo- 

 logical structure of the region mentioned. They do not 

 appear as lava flows, as subaerial eruptions, but as intru- 

 sive bodies forming dikes, necks, sheets or laccolites en- 

 closed in sedimentary rocks. It is not to be doubted that 



