ERUPTIVE ROCKS FROM MONTANA. 53 



show zonar structure, indicated by color or inclusions of 

 gas or groundmass. Twins (twinning plane coPco ) are 

 quite frequent. This augite is identical with the variety 

 occurring in the trachytes and described previously. 



The olivine is usually sharp-edged, clear and fresh, 

 sometimes surrounded by a narrow border of biotite ; when 

 decomposing a yellowish brown ferruginous serpentine 

 results. A mineral of the spinell group is observed as in- 

 clusion in the olivine. Magnetite is abundant and often 

 enclosed in the augite. 



The order of solidification has evidently been magnetite, 

 olivine, augite, the first being the oldest. Later than 

 the augite is the mineral determined as analcite. Together 

 with the other porphyritic cr^^stals it is imbedded in the 

 groundmass and appears as hexagonal, seldom octagonal, 

 most frequently simply rounded sections. In size they do 

 not exceed one millimeter but are frequently much smaller. 

 Most of the crystals are perfectly isotropic but not quite 

 clear, being somewhat clouded by minute interposi- 

 tions which under large magnifying power prove to be 

 largely gas, in part also glass inclusions. The former 

 have often a very irregular form. Irregular spots showing 

 a faint double refraction are sometimes noted, more so in 

 some sections than in others. Under favorable circum- 

 stances an imperfect cleavage in two directions, crossing 

 each other perpendicularly, may also be noticed. Minute 

 fragments from an exceptionally large crystal melt rather 

 easily and quietly before the blowpipe to a white enamel. 

 In one thin section a large crystal showing irregular oc-. 

 tagonal form with very distinct cleavage was selected for 

 experiment. It was uncommonly clear and perfectly iso- 

 tropic. Hydrochloric acid dissolves it easily upon very 

 slight heating under abundant formation of chloride of 

 sodium. Ignition does not make it opaque and does not 



