MOUNT RAINIER 



eastern slopes of Mount Rainier, during the seasons of 

 1895 and 1896, the writer had opportunity to make 

 some general observations on the rocks of this moun- 

 tain, and the petrographical material then collected 

 has since been studied. The observations and col- 

 lections were of necessity limited, both by the recon- 

 naissance character of the examination and by the 

 mantle of snow and ice which covers so large a part of 

 this volcanic cone. 



Two classes of rock are to be discussed as occurring 

 on Mount Rainier : the lavas and pyroclastics which 

 compose the volcanic cone and the granitic rocks 

 forming the platform upon which the volcano was 

 built up. 



VOLCANIC ROCKS 



GEOLOGIC RELATIONS 



On Crater Peak a dark line of rock appears above 

 the snow, and here the outer slope of the crater rim is 

 found to be covered with blocks of lava. A black, 

 loose-textured andesite is most abundant, and from 

 its occurrence on the edge of this well-defined crater 

 may be regarded as representing the later eruptions of 

 Rainier. Lower down on the slopes of the mountain 

 opportunities for the study of the structure of the 

 volcanic cone are found in the bold rock masses that 

 mark the apexes of the interglacial areas. Examples 

 of these are Little Tahoma, Gibraltar, Cathedral 

 Rock, the Wedge, and the Guardian Rocks. These 

 remnants of the old surface of the cone, together with 

 the cliffs that bound the lower courses of the glaciers, 

 exhibit the structural relations very well. 



Even when viewed from a distance these cliffs and 

 peaks are seen to be composed of bedded material. 

 Projecting ledges interrupt the talus slopes and express 

 differences of hardness in the several beds, while 

 variations in color also indicate separate lava flows 

 and agglomeratic deposits. Gibraltar is thus seen to 



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