308 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1928 



unite southward into one great field of lava, lonel}' and uninhabited. 

 Black Top Butte, the farthest in this march of cones, lies approxi- 

 mately 11 miles southeast of Big Cinder Butte. Still farther away 

 are yellow grass-covered cones, and in the distance are barely dis- 

 cernible the snow-covered Portneuf and Bannock Mountains. Four 

 miles to the south lies a chaos of cinder crags and jagged lava sur- 

 rounding a high cinder cone called North Laidlow Butte. Half a 

 mile beyond this cone is Little Laidlow Park, a grass-covered field 

 of ancient lava, appearing pale yellow under the bright desert sun 

 of midday. Farther away on the plain are low lava domes, and in 

 the far distance beyond the Snake River are pale blue mountains 

 along the Idaho-Utah line. 



To the w^est for about 6 miles the lava has flowed against the 

 southern spur of the White Knob Mountains, filling the valleys as if 

 they were bays and leaving the ridges like projecting headlands in 

 a black sea. To the northwest there are crater pits, spatter cones, 

 cinder cones, and lava flows massed along the Great Rift. Many of 

 the cinder cones are brilliant red at noon, but slowlj^ change to 

 purple with sundown. On the tops of many of them are crater pits 

 which are especially beautiful under the lengthened purple shadows 

 of evening. Beyond these features rise the granitic White Knob 

 Mountains covered with grass and streaked with small groves of 

 aspen in the twisted ravines and canyons. Farther northward are 

 the snow-clad Sawtooth Mountains. 



Before any lava had appeared in southeastern Idaho, the White 

 Knob Mountains of granitic rocks, then higher, projected south- 

 ward into the wide open valley of the ancestral Snake River. All 

 the rivers — Big Lost River, Little Lost River, Birch Creek, and all 

 the other streams adjacent to the area — flowed southward from the 

 mountain canyons and joined the ancient Snake. To-day these 

 rivers sink into the lava plain at the foot of the mountains and find 

 their way underground into the Snake River through numerous 

 crevices and cavities in the lava. 



The White Knob Mountains and their foothills w^ere undisturbed 

 except by stream erosion until a fissure called the Great Rift opened 

 on their slope. With this earth rifting there occurred an eruption 

 that built up a long line of cinder cones, and lava flowed southward 

 and eastward, cooling into thick jagged flows consisting largely of 

 broken blocks. Frozen in some of these lava blocks one can find 

 both large and small fragments of granodiorite that were stopccl 

 off from the roof of the lava reservoir and floated upw^ard in the 

 molten lava during the eruption. The lava must have issued at a 

 temperature of about 1,200° C, which was too low to remelt the 

 granitic inclusions; hence the white fragments retain all their 

 original characteristics. Numerous eruptions occurred during this 



