EXTINCT VOLCANO — ATWOOD 319 



no secondary cone had come into existence wlien the early glaciers 

 were present. Any more definite conclusions must await further 

 study of the volcanic history of the region. 



THE SIGNIFICANCE 



A glacio-volcanic sequence similar to that so clearly recorded in 

 the rocks surrounding Crater Lake has probably occurred in the 

 history of Mount Rainier, Mount Hood, Mount Shasta, and other 

 volcanoes of the Cascade Range, but the evidence is not visible. 

 Although Mount Rainier is dormant at present, the glaciers on its 

 slopes are always in danger of being destroyed by a renewed volcanic 

 eruption. Thus, previous generations of glaciers may have been 

 destroyed. If the summit of Mount Rainier were suddenly to dis- 

 appear, leaving a giant caldera in the base of the mountain, many 

 of the conditions observed today at Crater Lake would very prob- 

 ably be duplicated. The collapse of Mount Mazama opened a book 

 for the geomorphologist which might otherwise have remained 

 closed forever. 



From the standpoint of the glaciologist and vulcanologist the 

 results of the Crater Lake study are not limited in application to the 

 Cascade Mountain region but apply to all volcanic areas where the 

 summits rise to sufficient altitude to allow glaciers to form or where 

 previous conditions have favored glaciation. Assuming that the 

 stages in the growth of Mount Mazama preceding its collapse were 

 normal for high volcanoes during the Pleistocene, we should expect 

 to find similar glacio-volcanic sequences in many of the mountain 

 regions throughout the world. Unfortunately, the records of other 

 volcanoes are so completely hidden that a sequence comparable to 

 that discovered at Crater Lake may never be available for study. 

 Evidence of glacio-volcanic sequences may, however, be uncovered 

 in the walls of the stream valleys which radiate from large volcanoes. 

 It is very probable that the valleys which dissect the lower slopes of 

 Mount Rainier hold records of this kind. 



In addition to the light which Mazama's story may cast upon the 

 history of other volcanoes, there is a still greater significance. Each 

 glacial deposit exposed in the rim indicates that snow accumulated 

 on the mountain and that ice fields developed of sufficient size to 

 descend the slopes, polishing and deeply striating the rocks. The 

 time required to accomplish this task is necessarily great, probably 

 many thousand years. The four known glacial records interbedded 

 with volcanic materials indicate four long intervals in the growth of 

 Mount Mazama. 



The development of a soil horizon like that at Pumice Point re- 

 quired much time. Each layer of volcanic breccia overlain by a 



