FKOZEN SEAS OF LAVA. 93 



originally not less than four thousand feet above the base of 

 this astounding lava-deposit. 



This vast and ponderous sheet of lava appears to have 

 flowed through fissures from the Cascade Mountains, and nat- 

 urally to have accumulated to greatest thickness along that 

 range. The sheet extends across eastern Oregon to the Blue 

 Mountains. From this range, also, other, but less copious, 

 lava streams were poured forth. 



The chain of volcanic outbursts continued southward into 

 the Sierra Nevada. The lava vents here were more local and 

 isolated. The lava, though enormous in quantity, was less 

 than in Oregon, and overspread the surface less generally. 

 Under these circumstances, great volcanic cones were built 

 up such as Lassen, Shasta, Hood, and Ranier. From 

 Lassen's Peak the sheets of lava form a regular slope to the 

 Sacramento river. Through this the streams have cut their 

 channels five hundred to eight hundred feet deep. 



Nearly all the so-called Basin Ranges lying eastward of 

 the Sierra Nevada, through Utah and parts of California and 

 Arizona, are composed, at least in part, of ancient lavas. 

 Through the Plateau Region, farther east, lavas are equally 

 abundant. In the Sevier Basin, according to Gilbert, the 

 great Sevier fault, or break, through the rocks, exposes a 

 maximum thickness of two thousand feet. South of the Col- 

 orado is a much larger lava-basin, spreading several broad 

 lobes over into New Mexico, the most easterly of which, 

 reaches nearly to the Rio Grande. Its extreme limits are 

 three hundred and twenty-five miles apart. It includes the 

 San Francisco, Mogollon, and Sierra Blanca mountains. This 

 outflow proceeded from a large number of vents. In San 

 Francisco Mpuntain we have a pyramid of compact lava 

 nearly five thousand feet high, with slopes of ten to twenty 

 degrees. 



The examples cited are sufficient to impress the imagina- 

 tion and enable us to appreciate the magnitude of the work 

 of heat in the geological aeon not long antecedent to the dawn 

 of modern times. We mark the Tertiary, and especially the 



