MANSFIELD AND LARSEN : NEPHELINE BASALT 463 



GEOLOGY. — Nepheline basalt in the Fort Hall Indian Reserva- 

 tion, Idaho.^ George R. Mansfield and Esper S. Lar- 

 SEN, Geological Survey. 



occurrence^ 



One of the igneous rock samples collected by the United 

 States Geological Survey party in the Fort Hall Indian Reser- 

 vation, Idaho, in the summer of 1913, proved upon examination 

 in thin sections to be nepheline basalt. Because of the relative 

 rarity of rocks of this type any new occurrence of them is of scien- 

 tific interest and is worthy of note. Indeed, nepheline-bearing 

 rocks and all the so-called alkali rocks are very rare on the 

 Pacific slope of North America. 



The Fort Hall Indian Reservation is in southeast Idaho and 

 may be conveniently reached from either Pocatello or Blackfoot 

 on the Oregon Short Line Railroad. Pocatello lies about 3 miles 

 south of the central part of the reservation and Blackfoot is on 

 the northern border. The locality of the nepheline basalt is 

 in the northeast part of the reservation about 24 miles nearly 

 due east of Blackfoot. The rock forms a little knoll on a ridge 

 that descends northeast along the north side of Wood Creek in 

 the SW.i of the SE.i sec. 18, T. 3 S., R. 38 W. Boise meridian. 

 The area occupied by the basalt is small and the occurrence would 

 be insignificant were it not for the unusual mineralogical compo- 

 sition of the rock. 



The rocks in the immediate vicinity of the basalt are mainly 

 sedimentary rocks of Triassic and possibly Jurassic age that out- 

 crop in bands extending from northwest to southeast and appar- 

 ently form the west limb of a syncline that is inclined northeast. 

 The eastern limb of the syncline is more or less concealed by 

 Tertiary sediments and volcanic rocks. The sedimentary rocks 

 are somewhat faulted. The knoll on which the basalt occurs lies 

 in the belt of Portneuf limestone, a massive and siliceous lime- 

 stone that constitutes the upper formation of the Thaynes group 

 (Lower Triassic). About one-third mile to the northeast are 

 massive ledges of the Higham grit, the basal member of the Nug- 



' Published by permission of the Director of the U. S. Geological Survey. 

 2 By Mr. Mansfield. 



