National Geographic : 1897 Feb
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 lake appears along the inner slope directly north of Wizard island, and is locally known as the Devil's 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 upon 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 glaciation 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 Mount 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 glaciation of the rim is such as to re quire a large peak for its source.