206 



SCIENCE. 



[N. S. Vol. XV. No. 371. 



i-ather abruptly in places upon its borders, 

 especially to the westward, where the un- 

 derlying Cretaceous and Tertiary sedi- 

 ments come to the surface. It is sur- 

 mounted by volcanic cones and coulees, 

 which are generally smooth but sometimes 

 rough and rugged. The cones vary greatly 

 in size and are distributed without regu- 

 larity. Each has been an active volcano. 

 The fragments blown out by violent erup- 

 tion have fallen about the volcanic orifice 

 from which they issued, and built up cinder 

 cones. From their bases have spread 

 streams of lava, raising the general level 

 of the country between the cones. From 

 some vents by many eruptions, both ex- 

 plosive and effusive, large cones, like Pitt, 

 Shasta and Hood, have been built up. 

 Were we to examine their internal struc- 

 ture, exposed in the walls of the canyons 

 carved in their slopes, we should find them 

 composed of overlapping layers of lava and 

 volcanic conglomerate, a structure which is 

 well illustrated in the base of Mt. Mazama. 



VIEW OF MT. MAZAMA FEOM A DISTANCE. 



Approaching Crater Lake from any side 

 the rim by which it is encircled, Mt. Ma- 

 zama, when seen at a distance, appears as 

 a broad cluster of gentle peaks rising about 

 a thousand feet above the general crest of 

 the range on which it stands. The topo- 

 graphic prominence of Mt. Mazama can be 

 more fully realized when it is considered 

 as the head of Rogue River and sends large 

 contributions to the Klamath River, besides 

 being close to the head of the Umpqua. 

 These are the only large streams breaking 

 through the mountains to the sea between 

 the Columbia and the Sacramento, and 

 their watershed might be expected to be 

 the principal peak of the Cascade Range. 



GENERAL VIEAV OP MT. MAZAMA AND ITS 

 LAVAS. 



Arriving by the road at the crest of Mt. 

 Mazama, the lake in all its majestic beauty 



appears suddenly in view and is pro- 

 foundly impressive. The long gentle slope 

 upon the outside at the crest is changed to 

 a precipice. Nearly 20 miles of irregular 

 cliffs ranging from 500 to nearly 2,000 feet 

 in height encircle the deep blue lake and 

 expose in sections many streams and sheets 

 of lava and volcanic conglomerate which 

 radiate from the lake as a center. Along 

 the southern border the rim above the lake 

 level has many superimposed flows, but 

 upon the northeast where it is not so high 

 it is composed largely of one great flow 

 which coursed down a ravine of the ancient 

 Mt. Mazama. 



The rim is cut by a series of eleven dikes, 

 one of which is prominent and reaches 

 from below the lake level to the rim crest. 

 Others rise only part way and spread into 

 flows for which they afforded an outlet. 

 Near the west border of the lake is Wizard 

 Island with its lava field and cinder cone 

 surmounted by a perfect crater. 



Three kinds of lava occur in Mt. Mazama, 

 andesite, dacite* and basalt. The andesites 

 form nearly nine-tenths of the mass of 

 the rim. Dacites, generally accompanied 

 by pumice, form the surface flows upon 

 the north and east crest of the rim and are 

 everywhere underlain by andesites. Both 

 came from the central vent of Mt. Mazama, 

 which, however, furnished no basalt. It all 

 came from a number of small volcanic 

 cones upon the oujter base of the mountain. 

 The dacites are younger than the basalts, 

 for showers of dacite pumice fell in the 

 extinct craters of the basalt cones. As the 

 oldest lavas of Mt. Mazama are andesites, 

 so are the latest, for the lava of Wizard 

 Island is andesite which was poured out 

 upon the floor of the caldera after the de- 

 struction of Mt. Mazama. It marks the 

 beginning of a second petrographic cycle 

 from the same vent. 



* My collections were studied by Dr. H. B. Pat- 

 ton, who now regards as dacites what I have here- 

 tofore called rhyolites. 



