18 LAKES OF NORTH AMERICA. 



Another instance of the formation of a hike on acconnt of the filling 

 of a valley by a lava flow, hut on a much smaller scale than the example 

 cited above, has been observed by the writer, at the junction of Canadian 

 and Mora rivers, New Mexico. Canadian river, for a distance of perhaps 

 a hundred miles, flows through a steep-walled gorge, in which for a space 

 of several miles, near ^v']lere ]\Iora river joins it, there is an inner gorge, 

 as indicated in the followintr cross section : 



Fig. 1. — Cross Sectiox of the Canons of Canadian and ISIora Rivers, New Mexico 



(J. J. Stevenson). 



The valleys excavated by Canadian and Mora rivers were filled to a 

 depth of 400 feet by basalt, as indicated by vertical lines in the section, 

 and were subsequently eroded to .a depth of 230 feet deeper than before 

 the obstruction. The lake which existed above the lava flow has been 

 drained, and only indefinite traces of its former presence now remain. ^ 



Similar instances of the damming of streams by lava flows, are known 

 on the west slope of the Sierra Nevada, but are also of ancient date. 

 The lakes that were formed have been drained, and their bottoms trans- 

 formed into grassy valleys. 



Two small lakes, held in check by a recent lava stream, now exist at 

 the Cinder cone, near Lassens peak, in northern California. Beneath the 

 lava retaining these lakes there is a sheet of fine lacustral marl and dia- 

 tomaceous earth, showing that a former lake was partially filled by the 

 molten rock, now hardened into compact basalt.^ 



Another class of lakes due to volcanic agencies, occupy the bowls of 

 extinct craters. These occur in various situations, being sometimes at 

 the summits of high volcanic cones, and again in depressions in broad, 

 featureless plains. The walls enclosing them are sometimes formed of 

 compact lava, but more frequently consist of scoria, lapilli, and so-called 

 ashes, blown out of volcanic vents during periods of violent eruption. 



1 This instructive locality has been described by J. J. Stevenson, in Am. Phil. Soc, Proc, 

 1880, pp. 84-87. 



^ J. S. Diller. " A Late Volcanic Eruption in Northern California." U. S. Geol. Surv., 

 Bulletin No. 79, 1891. 



