REVIEWS. 637 



be analcite. Biotite is sometimes present in small quantities. From 

 the very fresh appearance of these rocks it seems probable that the 

 analcite is a primary crystallization from the molten magma. The 

 groundmass of the rock consists of augite and smalL_crystals of anal- 

 cite with magnetite. Mr. Lindgren calls attention to the difficulty of 

 distinguishing glass, if present, from isotropic analcite. 



In the Bear Paw mountains there are dikes of rocks related to those 

 just described and which correspond to the lamprophyres of Rosenbusch. 

 They are dark, fine grained, and porphyritic with phenocrysts of augite 

 and long flakes of brown mica. The groundmass consists mostly of 

 lath-shaped plagioclase, augite and mica. Some varieties with pheno- 

 crysts of olivine and augite, in a glassy groundmass without feldspar, 

 approach certain limburgites. 



The paper by Messrs. Wolff and Tarr is confined to a description 

 of certain trachytic and syenitic rocks in the Crazy mountains. The 

 first notice of the interesting rocks of this locality was published by 

 Mr. Wolff in 1885, and he has since undertaken a much more exten- 

 sive investigation of the same group of rocks, which is not yet com- 

 pleted. The trachytes form dikes, sheets and laccolites in the northern 

 portion of the range, and are associated with theralite. Like the 

 theralites and some other rocks of this range, they are coarse grained, 

 almost granitic when in thick sheets, fine grained and porphyritic 

 in the smaller sheets, dikes, and apophyses. When occurring in the 

 latter forms the rocks have a trachytic habit, and are called acmite- 

 trachyte. The phenocrysts are glassy feldspar, augite and small 

 sodalites. Biotite is scarce. The feldspar is soda-microcline or 

 anorthoclase. The augite is pale green at the center, and becomes 

 dark green at the margin, where the optical characters are those of 

 aegirine, similar to that in the theralite. The groundmass consists 

 essentially of lath-shaped feldspar and acicular crystals of aegirine. 

 With the green aegirine a few brown needles of acmite occur. There 

 is a variable amount of interstitial matter between the feldspars of the 

 groundmass which is probably nepheline in part, and partly analcite, 

 derived from the alteration of the nepheline. 



The coarse grained forms of the rock, or syenite, consist of the 

 same essential minerals as the trachytic varieties. Sodalite is rare in 

 the coarse rocks, and acmite is not always present. Chemical 

 analyses of these rocks are published, but the discussion of them is 

 postponed until the monograph of the whole group of rocks is pre- 



