238 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



BASIC BRECCIA AND LAVA FLOWS. 



The basic breccia is, with some exceptions, dark colored, gray, and 

 reddish brown, and is as a whole basaltic. Variations in mineral composi- 

 tion occur within narrow limits, and are most noticeable in the amounts 

 of olivine and hypersthene. All proportions ot olivine, exists. According 

 to the preponderance of one or the other the rocks may be classed as 

 basalts or pyroxene-andesites, though their other characters remain much 

 the same. Various modifications of the rock are mingled in the breccias, 

 but to a different extent in different localities. In some cases the material is 

 very uniform in its habit. Of the specimens collected two-thirds are basalt. 

 Hornblende-bearing varieties are extremely rare, and occur in the neighbor- 

 hood of the early acid breccia. 



In most instances the rocks exhibit no large phenocrysts, but carry a 

 multitude of minute tabular feldspars and somewhat larger pyroxenes, with 

 more or less olivine. There are modifications of the rock — which are more 

 numerous within the region of chaotic breccia — that carry tabular feldspars 

 5 to 8 mm. long, and still others with the same form of feldspar 30 mm. 

 long, the large feldspars being crowded with inclusions. The rocks are 

 very generally vesicular and scoriaceous, but a part are dense and compact. 

 In thin sections the groundmass possesses an andesitic habit, and consists 

 of brown and red globulitic glass, which is occasionally colorless, with 

 microlites of feldspar and grains of pyroxene and magnetite. In some 

 instances it is opaque through an excess of iron oxide, and in other cases it 

 is holocrystalline. The phenocrysts are plagioclase, augite, magnetite, and 

 more or less hypersthene and olivine. The microscopical characters of 

 these minerals are the same as in other occurrences in this region. A basalt 

 with andesitic habit from Saddle Mountain is shown in PL XXXIV, fig. 3. 



Hypersthene is more abundant as olivine is less so, and is absent from 

 the rocks with much olivine. Occasionally hypersthene is surrounded by 

 augite with parallel orientation. In some cases augite is brown at the 

 center, with a zonal structure. In most cases it is pale green. The olivine 

 is unaltered in many occurrences and completely serpentinized in others. 

 In general the rocks are very fresh, with slight indications of weathering, 

 and only an occasional development of zeolites An unusual and interesting 

 variety of the latter mineral was collected and investigated by Prof. L. V 



