THE ROCKS OF MOUNT RAINIER 



morphic, the hornblende retaining its characteristic 

 outlines, but often there has been resorption. In 

 one andesite the abundance of these remnants of 

 hornblende and also of augite anhedrons in the ground- 

 mass may justify the conclusion that this augite 

 andesite is of derivative origin, of the class described 

 by Washington. 1 It may be noted also that hyper- 

 sthene shows a tendency to magmatic alteration, 

 although only rarely. 



In a basal flow in Moraine Park, the slaggy and 

 compact phases show differences in phenocrysts as 

 well as in ground mass. The glassy rock has hyper- 

 sthene as the predominant phenocryst, while feldspar 

 is the more important in the compact and more crys- 

 talline andesite. 



The distribution of the rock types described above 

 is of interest. On the northern slope of the mountain, 

 between Willis and Carbon glaciers, the characteristic 

 lava is a gray andesite, smooth to rough in texture, 

 and showing platy and columnar parting. Hyper- 

 sthene is not the prevailing pyroxene, and olivine is 

 usually present, often in such abundance as to make the 

 rock a basalt. 



In Moraine Park gray andesites also predominate, 

 with both pyroxenes as phenocrysts, but here hyper- 

 sthene is the more important. On the eastern slope 

 on the Wedge, between Winthrop and Emmons 

 glaciers, the lavas are pyroxene-andesites and vary 

 much in megascopic appearance, although little in 

 microscopic characters. These rocks are quite dis- 

 tinct from any seen to the north. The nunatak in 

 Emmons Glacier is composed of hypersthene-andesite, 

 but on Little Tahoma the lava shows more variety. 

 Both augite-andesite and hypersthene-andesite occur, 

 while at the southern end of this interglacial rock mass, 

 just east of Cowlitz Glacier, the cliffs are composed of 

 the prismatic black basalt. On Crater Peak, and 



1 Jour. Geol., Vol. IV, 1896, p. 276. 



249 



