638 THE JOURNAL OF GEOLOGY. 



pared. The resemblance between certain features of the rocks of 

 Montana and those from Arkansas, described by J. Francis Williams, 

 is pointed out by each of the writers cited. The resemblance to the 

 lamprophyric rocks in the Absaroka range, Wyoming, 'east of the 

 Yellowstone National Park, is also noticed. 



Some of the petrographical characteristics of the rocks of this 

 region are : The prevalence of orthoclase in many intermediate and 

 basic rocks, leading to the frequent occurrence of trachyte and syenite 

 and some forms of lamprophyre, as well as* its presence in prominent 

 crystals in the andesites and porphyrites, and the frequent occurrence 

 of dark green augite and aegirine, and occasionally of acmite. 



The difficulty of distinguishing colorless glass from isotropic anal- 

 cite, both of which may occur in certain varieties of lamprophyre, 

 makes it necessary to use the greatest care in determining the charac- 

 ter of the apparent base in these forms of rocks. It seems probable 

 to the reviewer that in some instances, in which an amorphous glass 

 has been described as forming the matrix of the microscopic crystals 

 in some lamprophyric dike rocks, it will be found that a definite 

 isotropic alkali mineral is present, and that the rock is holocrystalline 



Joseph P. Iddings. 



