642 ERUPTIVE ROCKS FROM MONTANA— MERRILL. vol.xvil 



section tliey are of a liglit-greenisli color. The porphyritic plagioclases 

 are small aud widely scattered. 



As already noted, the rock is quite vesicular, the vesicles being wholly 

 or in part filled by white, dull red, and greenish zeolites. There is also 

 a smoky-brown, undetermined mineral which occurs only as a narrow 

 border of minute radiating: fibers projecting inward from the cavity 

 wall and visible only with the microscope. The white mineral is by far 

 the most abundant of all the secondary constituents. When viewed 

 in the section and between crossed nicols this is in some cases quite 

 isotropic, and in others polarizes faintly in dull colors, the field being 

 divided into polygonal areas over which the shadows play, alternately 

 as the stage is revolved. The appearance is such as to suggest at 

 once the anomalous aualcite described by Ben-Saude,* although in 

 the present case the optical peculiarities are less pronounced. An 

 examination of the hand specimen reveals in the larger cavities many 

 small, nearly colorless trapezohedra of the mineral which have a specific 

 gravity of 2.7, as determined by a Westphal balance, and which fuse 

 quietly to a clear, colorless glass at 2.5 of Dana's scale. These charac- 

 teristics demonstrate the mineral as aualcite beyond doubt. The dull- 

 red zeolite is quite colorless and isotropic in thin sections; examined 

 in the hand specimen, with a pocket lens, it shows a rhombohedral 

 cleavage, and the small si^linters obtainable were found to give the 

 blowpipe reactions of chabazite. ■ Other of the amygdules, from 1 to 

 3 mm. in diameters are filled by a hard and very brittle dull, dark-brown 

 mineral which always breaks away during the grinding of the section, 

 but which gives blowpipe reactions for hematite. 



Hornblende andesite. — From small outcrops on ridge east of Fort 

 Ellis. This is a compact, light-gray rock (IS'o. 62400, U.S.KM.) with 

 macroscopic brown hornblendes and whitish feldspars. Under the 

 microscope it shows a compact groundmass of feldspar microlites and 

 opacite grains carrying abundant porphyri tic hornblendes, plagioclases, 

 and smaller light- green augites. The hornblende is by far the most 

 abundant of the poiphyritic constituents, and is readily recognizable 

 by its well-defined crystallographic outlines, though in nearly every 

 case its substance has completely changed to the characteristic opacite 

 granules. The plagioclases are very muddy through impurities and 

 decomposition. The most interesting feature of the rock is the abun- 

 dant sprinkling of large brick-red pleochroic axDatites, as shown in 

 figs. 1-7. These occur in all sizes up to 0.6 mm. The colors vary from 

 colorless through yellow to brick-red, the deeper color being due to 

 innumerable inclosures, which are represented by black dots in the fig- 

 ures. The distribution of the color is not uniform through the whole 

 mass of the crystal, but, as in figure 2, a crystal may be bright yellow at 

 one end and red at the other, or, as in figure 6, red in the center and 

 fading out gradually to colorless at the end s. In the cross section shown 



* Neues Jahrb. Vol. ii. 1882, p. 41. 



