306 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1935 



exjDosed in the rim surrounding the present lake indicate that four, 

 and possibly many more, stages of glaciation were interspersed with 

 the periods of vulcanism. This continued glacio-volcanic sequence 

 is suggested in figure 6. 



It will be noted from the sketches that the mountain at first had 

 but one cone, while during the later stages of volcano building a 

 secondary cone developed. This smaller mountain is Little Mazama. 

 The absence of this cone during the glacial stage illustrated by 

 figure 2 and its existence during later stages in the growth of the 

 mountain are determined by the direction of striae markings found 

 buried at different levels beneath lavas and glacial debris (see figs. 

 12 and 13). 



Without discussing at this place each of the glacio-volcanic se- 

 quences, we arrive at the stage in the growth of the mountain 

 represented by figure 4. The volcano was apparently dormant, and 

 large glaciers radiated from the summits of Mount Mazama and 

 Little Mazama. While these glaciers still existed, there commenced 

 a gradual destruction of the cone. Renewed volcanic activity pro- 

 duced a shower of pumice which whitened the entire landscape. 

 Many of the high points on the rim, as well as the morainic deposits 

 left by the last glaciers, are still partially buried beneath this pumice. 

 But in spite of the enormous amounts of this fine debris scattered 

 over the landscape, the disappearance of the entire mountain mass 

 above the dotted line in figure 4, and much of the core to a depth 

 of 3,000 to 4,000 feet below the present rim, cannot be explained by 

 explosive eruption alone. 



Approximately 15 cubic miles of material have disappeared in 

 order to produce the landscape shown in figure 5. If explosion 

 accomplished this great change, we would expect to find a thick 

 mantle of fragmental andesitic lava and breccia scattered over the 

 surrounding countryside. Furthermore, the glacial deposits, formed 

 by the glaciers which existed only as long as the mountain existed, 

 should be heavily buried with the fragmental material derived from 

 the destruction of the mountain. Instead we find very little angular 

 material in the region, and most of that had its origin prior to the 

 last glaciers which existed on the mountain before its destruction. 



The possibility that Mount Mazama never became a high vol- 

 canic cone was carefully considered in the field. If the mountain 

 had not exhibited evidence of recurrent glaciation, the existence of 

 a high cone would have been difficult to establish. However, since 

 glacial evidence is unmistakably present and, furthermore, since the 

 Pleistocene glaciers of the Cascade Range were restricted to the 

 higher mountains, it is logical to conclude that Mount Mazama 

 attained a height comparable to the peaks of the Cascade Range 



