44 CRATER LAKE, OREGON 



the molten material cools, the fissure becomes filled with solid 

 lava and forms a dike. The best example of this sort about 

 Crater lal^e appears along the inner slope directly north of 

 Wizard island, and is locally known as the Devirs Backbone. 

 This dike rock standing on edge varies from 5 to 25 feet in thick- 

 ness and cuts the rim from water to crest. Dikes are most 

 numerous in the older portion of the rim under Llao rock. 

 They do not cut up through Llao rock and are clearly older than 

 the lava of which that rock is formed. Dikes occur at intervals 

 all around the lake, and radiate from it, suggesting that the 

 central volcanic vent from which they issued must have been 

 Mount Mazama. 



There is another important feature concerning the kinds of 

 volcanic rocks and their order of eruption and distribution about 

 the rim of Crater lake that is of much interest to the geologist. 

 All the older lavas comprising the inner slope of the rim, espe- 

 cially toward the water's edge, are andesites. The newer ones 

 forming the top of the rim in Llao rock. Round Top, and the 

 Rugged Crest about the head of Cleetwood cove, as well as 

 at Cloud Cap, are rhyolites. Other later flows, all of which 

 escaped from the smaller adnate cones \x\)o\\ the outer slope of 

 the rim. are basalts. The eruptions began with lavas of medium 

 acidity (andesites), and after long-continued activity lavas both 

 rich (rhyolites) and poor (basalts) in silica follow, giving a com- 

 pleteness to the products of this great volcanic center that make 

 it an interesting field of study. Furthermore, the remarkable 

 opportunity afforded by the dissected volcano for the examina- 

 tion of its structure and succession of lavas is unsurpassed. It- 

 should be stated, before dismissing the kinds of lava, that there 

 are some rhyolites in the Sun Creek canyon south of the lake 

 that appear to be older than those upon the north side, and that 

 the final lava of the region on Wizard island is andesitic. 



The giaciation and structure of the rim clearly establish the 

 former existence of Mount Mazama, but there may well be doubt 

 as to its exact form and size. Judging from the fact that Mount 

 Shasta and the rim of Crater lake have the same diameter at an 

 altitude of 8,000 feet, and that their lavas are similar, it may 

 with some reason be inferred that INIount Mazama and Mount 

 Shasta were nearly of equal height. The slopes of Mount Shasta 

 may be somewhat steeper than those of the rim of Crater lake at 

 an equal altitude, but the giaciation of the rim is such as to re- 

 quire a large peak for its source. 



