THE KATMAI REGION, ALASKA 



575 



along a system of accompanying breaks. Such a relation appears 

 to be not uncommon in other volcanic districts. 



The Katmai group of volcanoes has evidently been built up on 

 a platform of Upper Jurassic sediments, and several features show 

 that comparatively recent flows have produced marked changes of 

 topography. For instance, the canyon of Katmai River (shown 

 in Fig. 4) is a narrow defile connecting open valleys above and 

 below. It is evident that a former open valley here was invaded 

 by floods of lava coming down from Mount Katmai, which shifted 



Fig. 5. — Lava flow of basic andesite overlying glaciated sandstone and a small 



remnant of till (at pick), at foot of the lava cliffs shown in Figure 4. 

 R. F. Griggs, 1919. 



Photograph by 



the river over upon the lower slopes of the Barrier Range. • The 

 river has again cut nearly to grade along a narrow canyon, and 

 beneath the lava-flows may be seen beds of till and sandstone 

 surfaces grooved and polished by glacial action (Fig. 5). In this 

 vicinity lava-flows of apparently post-glacial age measure 1,500 to 

 2,000 feet in thickness (see Fig. 4). 



The lavas of the group of cones that we are considering seem to 

 be predominantly basic andesites. In Mount Katmai itself the suc- 

 cession of flows that have built up the cone is now revealed in the 

 great crater pit, and they appear to be of medium to basic character. 

 The fragments of old rocks thrown out with the new lava in the 



