ERUPTIVE ROCKS EROM MONTANA. 45 



discovered with the naked eye. Microscopicallv, they 

 consist of idiomorphic biotite and hornblende with abun- 

 dant, hithhke. triclinic feldspar, some irregular grains of 

 the same and of monoclinic feldspar; the interstices be- 

 tween the latter are filled with quartz in smaller or larger 

 quantities.* 



II. AuGEFE Syenites, 



This small but interesting group is at present limited to 

 three occurrences, all in the form of dikes. 



1. In Silurian quartzites of Belt Creek, Little Belt 

 Mountains. 



2. In Jurassic (?) strata at the stage station near Drv 

 Fork, road from Barker to Fort Benton". Little Belt 

 Mountains. 



3. In Cretaceous or Laramie strata at road, north side 

 of Main Pass, High wood Mountains. 



Mineralogically the augite syenites consist principally of 

 orthoclase, plagioclase. biotite and a p3a-oxene, probably 

 malacolite. As to structure thev are hyphidiomorphic 

 granular. 



The first contains small prisms of light green malacolite 

 and small, sometimes hexagonal biotite foils imbedded in 

 a coarser allotriomorphic granular mass of feldspar, ap- 

 parently orthoclase, sometimes twinned according to the 

 Carlsbad law, but more frequently in single grains. The 

 interstices between the grains are sometimes filled with 

 quartz. 



The second is a fine grained, light colored rock, in which 

 without lens may be seen black biotite foils and lathlike 

 feldspar crystals. Under the microscope straight long bi- 

 otite foils and slender apatite prisms appear as products of 



^Similar rocks have also been described by Mr. E. Wolff in " Notes on 

 the Geology of the Crazy Mountains," Northern Transcontinental Survey, 

 i88s. 



