ERUPTIVE ROCKS FROM MONTANA. 5 1 



basalts. West of Fort Benton, however, they are much 

 more frequent and were noticed to be especially well de- 

 veloped in the Sun River district and in the Birdtail 

 Mountains. Thev occur as large dikes or necks, prob- 

 ably also as intruded sheets and are wonderfully well ex- 

 posed by erosion. 



The basalt from Highwood Mountains is a dark green 

 tine-grained rock, in which dark green pyroxenes and 

 brown specks of olivine are visible with the naked eye. 



Thin scrfioii: Idimorphic augite, light green and con- 

 taining many glass inclusions; olivines, sharp-edged and 

 decomposed; no porphyritic feldspar ; groundmass hyalo- 

 pilitic with long feldspar needles, seldom striated and ex- 

 tinguishing about parallel to their longer axis ; contains 

 also small, irregular grains of augite. 



The basalts from Sun River and the Birdtail Mountains 

 are characterized by the same long prismatic augite of a 

 lighter or darker green color which is found in the High- 

 wood dike rocks. Olivine is not always present. The 

 groundmass is usually clearly basaltic, often holocrystal- 

 line and quite coarse. In a rock from Table Mountain 

 near Sun River, I thought that the presence of leucite was 

 probable. Re-examination of the slide does not seem to 

 confirm this. 



V. Analcite- Basalts. 



The most interesting group of the rocks from the High- 

 wood Mountains is doubtless that which is described in 

 the paper inVol. xv, loth Census, as Analcite (Nosean) 

 Basalts. They occur as dikes of varying dimensions in 

 the Cretaceous or Laramie sandstones of the Highwoods. 

 probably also as volcanic necks, together with augite sv- 

 enites, trachytes and plagioclase basalts; mineralogicallv 

 the}' consist of augite, olivine, magnetite, and a mineral 

 determined as analcite ; biotite is sometimes present in 



