294 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



In some glassy varieties the glass is brown in very thin sections, and 

 the microlites of colorless feldspar and pale-green augite are distinctly 

 contrasted with the glass. In other parts of the same rock the pyroxene 

 crystals, small and large, are reddened on the surface and have a red 

 opaque margin or incrustation, the smallest crystals being reddened 

 throughout, the large ones only marginally. In these parts of the rock the 

 glass is more often colorless, as though the brown pigment had been segre- 

 gated about the pyroxenes. But there are cases where reddened pyroxenes 

 occur in brown glass. In one rock the glass is yellow and orang-e in thin 

 section, and the pyroxenes are reddened and there is a reddish opaque 

 border to many crystals. This is more pronounced about the hypersthenes 

 than about the augites. Brown and yellow glasses grade into one another 

 in one rock. One variety has a beautiful brown globulitic glass base, with 

 the usual microlites. Another groundmass consists of colorless glass, 

 crowded with very minute thin prisms of pyroxene, feldspar, and magnet- 

 ite. The form of these crystals may be observed when a thin layer of 

 groundmass wedges out over a large crystal of feldspar. In thicker layers 

 the groundmass appears as a gray felt of these crystals. In the few holo- 

 crystalline modifications the microstructure seems to be that which would 

 be caused by the crystallization of the feldspar microlites against one 

 another. Their outline is lost, and all of the coloring matter of the glass 

 base is concentrated in the ferromagnesian constituents. Sometimes these 

 are distinctly formed pyroxenes, together with magnetite. In other cases 

 there is considerable chloritic or serpentinous material scattered among the 

 feldspars. In several instances the groundmass contains amygdules of 

 what appears to be chalcedony, sometimes coating and inclosing crystals 

 of tridymite. The chalcedony accompanies the opalization of hypersthene, 

 a form of alteration noticed by Kiich 1 in the anclesites of Colombia. In 

 general the rocks collected are almost free from decomposition of any kind. 



pyroxene-andesite. — These andesites have essentially the same habit as the 

 hornblende-pyroxene-andesites, but are darker colored as a whole. The 

 abundant small phenocrysts are lime-soda feldspar, hypersthene, and augite. 

 In a few cases there is a small amount of hornblende with black border, or 



1 W. Reies and A. Stiibel. Reisen in Siid-Amerita. Geologische Studien in der Republik Colom- 

 bia. I. Petrographie. I. Die vulkanischen Gesteine. Bearbeitet von Richard Kiich. Berlin, 1892. 

 Reviewed in Jour. Geol., Vol. I, No. 2, 1893, pp. 164-175. 



