ABSTRACTS 219 



inundation reached the vicinity of Clinton, and they mark the close 

 for that district, of the post-Mississippian erosion period, during which 

 the entire Pottsville series, reaching at some points a thickness of two 

 thousand feet or more, and a portion of the Lower Productive Coal 

 Measures were laid down in the Appalachian basin. 



Crater Lake , Oregon.^ By J. S. Diller. 



Crater Lake of southern Oregon is deeply set in the hollow base 

 of a larre cone upon the summit of the Cascade range. Its rim rises 

 bv moderate slopes a thousand feet above the general level of the 

 range, and the descent within to the lake is precipitous. The lake, 

 with an altitude of 6239 feet above the sea, has no outlet. It is 

 approximately circular, with an average diameter of about five miles, 

 and is completely surrounded by cliffs ranging from over 500 to 

 nearly 2000 feet in height. The steep slopes continue beneath the 

 water to a depth of 2000 feet. The great feature of the region is not 

 the lake, but the caldera which it half fills and thus partly conceals 

 but greatly beautifies. The rim has the structure of the peripheral 

 portion of the base of a great volcano. It is composed of lava streams 

 and sheets of volcanic conglomerate radiating from the lake. Sections 

 of the coulees appear upon the inner slope of the rim where their 

 broken ends form cliffs toward the lake, and it is evident that they 

 once converged, forming a large volcano on the site of the lake, from 

 which they issued. In some cases, as for example under Llao Rock, 

 the valleys filled by great flows upon the outer slope of the central 

 volcano are clearly visible in the section afforded by the rim. The 

 earlier lavas of the rim are andesites, and the later ones rhyolites, while 

 basalts, which are also of late eruption, are confined to small adnate 

 cones low down upon the outer slope of the rim. The rim is cut at a 

 number of points by dikes radiating from the lake, and this feature 

 taken in connection with the succession of lavas, and especially the 

 structure of the rim, clearly points to Crater Lake as a great volcanic 

 center. The rim of the lake has been deeply scored by glaciers, but 

 this phenomena is confined wholly to the outer slope. Striae and 

 moraines are found on the very crest of the rim overlooking the lake. 

 Deep U-shaped canyons extend directly through the rim ending on 



' Published in the American lournal of Science for March 1897, and in a more 

 popular form in the National Geographic Magazine for February 1897. 



