2 PROCEEDINGS OF THE IJATIONAL MUSEUM. vol.64 



LOCALITY. 



The specimens were received from Dr. Charles L. Kirtley, a physi- 

 cian of ChalHs, who is the original discoverer of the occiu-rence. 

 Some years ago Mr. Brown had found, in the low hills south of 

 Challis, some pebbles of colored corundum of such a quality as to 

 indicate the possible occurrence of gem material. \Miiie searching 

 for the corundum locality, which has not been rediscovered, Doctor 

 Kirtley found the mordenite. The writers wish to gratefully acknowl- 

 edge their indebtedness to Doctor Kirtley for his generosity in 

 supplying these interesting materials together with notes on their 

 place and mode of occurrence. 



Challis is an interior community in Salmon River Valley 58 miles 

 by stage northwest of Mackay, the nearest railroad point knd the 

 terminus of the Lost River branch of the Oregon Short Line Railroad. 

 The zeolites and other minerals are from a small area of amygdaloidal 

 rock in the low hills south of Challis in the wedge formed by Salmon 

 River and Antelope Valley and a short distance west of the Challis- 

 Mackay road. 



GEOLOGY. 



As described by Doctor Kirtley, the zeolite bearing formation 

 occupies an area 3 acres in extent, surrounded by low hills of yellowish 

 clay underlain by soft shale, the stratification being inclined about 

 20°. In this area the large masses of mordenite may be seen at 

 many places, although the specimens collected all came from a 

 single shallow cut. The shale hills are strewn wdth small pebble- 

 like masses of translucent quartz and chalcedon}^ A map accom- 

 panying a reconnaissance report by Umpleby ^ shows this area is 

 occupied by lacustrine sediments with included tuffs and flows of 

 andesite and basalt of probable Ivliocene age. The following descrip- 

 tion is quoted from this report. 



Lacustrine deposits occur in the northeastern and eastern portions of the 

 area studied. The northern area comprises an irregular belt about 10 miles 

 across with tongues reaching up the several valleys tributary to the basin in 

 which Challis is situated. To the south, west, and north the lake beds give 

 way to lava flows, but to the south they connect across a low divide at the 

 head of Antelope Valley with similar deposits in the valley of Big Lost River. 

 The area east of Bay Horse is of peculiar shape but seems to have an outward 

 continuation south of Antelope Ridge. 



Perhaps the best exposures of these beds are along the east bank of Salmon 

 River below Challis. Here the bluffs are about 200 feet high and, as seen from 

 across the river, are made up of light gray sandstones, probably tuffaceous, and 

 dove-colored shales. Conglomeratic members are sparsely and irregularly 

 distributed through the section. The beds vary in thickness from less than an 

 inch to several feet. In bluflfs north of Challis, tuffaceous material, generally 



' Joseph B. Umpleby. Some ore deposits in northwestern Custer County, Idaho. U. S. Geol. Survey 

 IJuU. 539, p. 27, 1913. 



