296 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



Basaltic andesite. — One of the fragments collected has a peculiar character. 

 It has a red glassy and microlitic groundmass with abundant small pheno- 

 crysts, in habit being similar to the rocks just described. But the porphy- 

 ritical minerals are augite, red mica with variable optic angle, numerous 

 serpentinized olivines, some red hornblende, and very few plagioclase feld- 

 spars. There are numerous microlites of plagioclase in the groundmass. 

 The rock is a more magnesian phase of the andesitic magma, related to 

 hornblende-pyroxene-andesite. It is not known in any large mass in the 

 region, although it must have existed as a lava flow at some period in the 

 history of the volcanoes of this range. 



Segregations are not often noticed, partly because they are easily over- 

 looked in the breccia, where the small fragments generally appear with 

 slightly different characters. In the massive bodies associated with the 

 breccia segregations sometimes occur. They are composed of crystals 

 about the size of the phenocrysts, either crystallized together in a hypidio- 

 morphic mass of plagioclase, hornblende, biotite, and pyroxene, with 

 magnetite, or combined as large crystals, with smaller ones forming a 

 relatively coarse-grained groundmass between (1652, 1667). 



LATE BASIC BRECCIA. 



The character and mode of occurrence of the basic breccia, which 

 overlies the light-colored and more siliceous breccia, have been indicated 

 in the paragraphs describing the late acid breccia; but they should be 

 restated in greater detail. In the vicinity of Mist Creek the basic breccia 

 overlies the light-colored breccia in several places, and is found to consist 

 of dark-colored pyroxene-andesites, sometimes with hornblende, sometimes 

 with olivine, or both (1484, 1486). On the south side of the summit of 

 Mount Chittenden, and on the ridges east and south, the dark-colored breccia 

 forms well-bedded accumulations whose layers are nearly horizontal and 

 overlie irregularly bedded or wholly imbedded light-colored breccia. The 

 upper breccia is composed of pyroxene-andesites, with more or less olivine, 

 and occasionally hornblende. This breccia probably extends down the 

 ridge to Lake Butte (1504 to 1507 and 1514, 1515). 



In the vicinity of Mount Humphreys the upper basic breccia overlies 

 the light-colored breccia in distinctly bedded accumulations which are 

 nearly horizontal. The breccia consists of dark-colored pyroxene-andesite, 



