ERUPTIVE ROCKS FROM MONTANA. 47 



rock was described in the paper mentioned as a " crystal- 

 line augite-trachyte." 



There is every reason to believe that these three dikes 

 are of the same age, presumably very late Cretaceous or 

 Post-cretaceous. At the two first-mentioned places the 

 rocks occur more isolated, while in the Highwood Mount- 

 tains the augite syenite is surrounded by a great number 

 of trachytic and basaltic dikes, also intrusive in the Cre- 

 taceous or Laramie strata and which cannot be much, if 

 any, younger than the former; in fact, a dike of the same 

 character as the augite syenite just described was noticed 

 by Mr. W. M. Davis cutting another dark green dike be- 

 longing to the later to be described basaltic 'group. 



in. 



RACHYTES. 



That normal porphyritic trachytic rocks of great variety 

 of appearance are abundantly represented in the High- 

 wood Mountains has been shown in my paper, frequently 

 referred to elsewhere. The ferro-magnesian silicates, ac- 

 companying the sanidine, are augite and biotite. While 

 the latter usually is present, it mostlv appears in smaller 

 quantities and is less conspicuous. The augite on the 

 other hand is always present and often verv prominent; 

 the dark green, octagonal, long and slender prisms, with 

 terminal P. OP are under the microscope of a more or less 

 intense green color, often somewhat pleochroitic and evi- 

 dently contain an admixture of the tegirine molecule. 

 This very characteristic augite is a seldom failing constit- 

 uent not only of the trachytes but also of the later to be 

 described basaltic dike rocks of Northern Montana. 



In this series of augite trachytes the relative quantities 

 of the two minerals — augite and orthoclase (sanidine) — 

 varies very much. At one end of the series stands a rock 

 composed nearl}^ entirely of feldspar; at the other end a 



