THE ROCKS OF MOUNT RAINIER 



be composed of interbedded lavas and pyroclastics, 

 and on the Wedge a similar alternation is several times 

 repeated, a pink agglomerate being exceptionally 

 striking in appearance. 



These lava flows and beds of volcanic ejectamenta 

 thus exposed dip away from the summit at a low 

 angle. The steepest dip observed was in the amphi- 

 theater at the head of Carbon Glacier, where in the 

 dividing spur the dip to the northeast is about 30. 

 Some exceptions in the inclination of the beds were 

 noted on the southeastern slope, where in a few cases 

 the layers are horizontal, or even dip toward the 

 central axis of the cone. In general, however, the 

 volcanics composing Mount Rainier may be said to 

 dip away from the summit at an angle somewhat lower 

 than that of the slopes of the present cone. In the 

 outlying ridges to the north, the Mother Range, 

 Crescent Mountain, and the Sluiskin Mountains, the 

 structure seems to be that of interbedded volcanics 

 approximately horizontal. The extent of the volcanics 

 from the center of eruption has not been determined. 

 Similar lava extends to the south, beyond the Tattoosh 

 Range, and volcanics of similar composition occur to 

 the north, in the Tacoma quadrangle. The latter 

 lavas and tuffs may have originated from smaller and 

 less important cones, now destroyed by erosion. 



A radial dike was observed at only one locality, 

 near the base of Little Tahoma. In several cases the 

 lava masses, as seen in cross section, are lens-shaped, 

 and where associated with fragmental beds have 

 unconformable relations. This shows that some of 

 the lava flows took the form of streams, relatively 

 narrow, rather than of broad sheets. Such a feature 

 is in accord with the distribution of rock types. Thus 

 along Ptarmigan Ridge for considerable vertical and 

 horizontal range the rock shows only slight variation. 

 The distribution of rock types will be more fully dis- 

 cussed in a later paragraph. 



243 



