BT.J0HN.1 REGION OF THE BLACKFOOT. 337 



lower level on the Avestern slope of the LighlaiKls, to bring the whole 

 into intelligible relation.ship. llence I have prefeiTcd to give a rather 

 full statement of tlie facts observed, which, in connection with the sec- 

 tion diagram, A\ill, I trust, be readily understood. 



Between two and three miles northwest of Higham's Peak, in the low 

 bluff-borders of the little stream which descends from this point into the 

 plain, an exi)osure reaching several hundred feet in thickness, consisting 

 of buff and reddish, thin-bedded sandstone was observed, dipping 35°, 

 N. 20° W. Half a mile lower doAvn the stream, a less extensive outcrop 

 of reddish sandstone shows an incbnation N. 35° E., at an angle of G0°. 

 These deposits are probably identical with the sandstones north of the 

 Coldwater, a couple of miles east of Fort Hall, mentioned on a preceding 

 page. They are here seen to form a low ridge of hills, which in a short 

 distance to the northward sinks beneath the high volcanic benches which 

 fill the debouchure of the Blackfoot and which occupy a wide belt ahmg 

 the eastern margin of the highlands descending to the Blackfoot. Wher- 

 ever these sandstones are exposed in this western border belt on the 

 margin of the plain they show unmistakable signs of confused disturb- 

 ance, which in certain localities seem to amount to non-conformity with 

 the Jurassic limestones in their immediate neighborhood. But not the 

 slightest evidence of an organic nature was detected by which their 

 age might be fixed, while, as above intimated, the other evidence is so 

 confused as to afford little light in so important a determination. 



To the southeast a group of low hills and upland basins intervene be- 

 tween the Blackfoot and the southern j^ortion of the highland divide 

 above described, which on the east show tilted Jurassic limestones and 

 sandy beds, inclining southwesterly, and which belong to the south- 

 western fiank of the system of folds just across the Blackfoot in the 

 vicinity of Stations XII and XIII, hereafter to be described, and which 

 are evidently intimately related to the disturbed belt between Stations 

 III and V. There are two princii^al upland swells, separated by shallow 

 basins which drain north into the Blackfoot, the western basin also 

 draining south into the Portneuf. The eastern basin is floored by basalt, 

 ■while the western one shows stretches of marshy, bitter flats, bordered 

 by low benches, with extensive spring deposits of calcareous tuta which 

 in places form quite extended benches in the borders at the foot of the 

 steei)er acclivities. The springs which built i\]) the latter deposits haA^e 

 become almost extinct, though a few feebly-flowing ones are still to be 

 seen. 



The eastern ridge is based on the Jurassic, which to the southward 

 probably rises uj) into more prominent ridges. But in this quarter the 

 bulk of its mass consists of the light marls and white limestones of the 

 Pliocene, which dip 20° to 25°, X. 30° E., the summits of the ridges, as 

 at Station XXX, being formed of cappings of trachyte and basaltic 

 lavas, dipping in the direction of the Blackfoot, or northeasterly at an 

 angle of 15° to 20°. Hence the tiltmg of these late Tertiary beds must 

 be attributed to forces acting elsewhere than in the belt of Jurassic folds 

 just over the Blackfoot in Station XIII. The light cream-color(id lime- 

 stones are i)erforated with the numerous molds of tresh- water gastropoda, 

 a species of Melania'^ being especially abundant and often the only fossil 

 found in these beds. Both in their lithology and organic remains, these 

 beds are unmistakably like deposits observed by Dr. Hayden, in 1871, 

 in the region of Great Salt Lake Basin to the south, and which are also 

 mentioned by Professor Bradley, who visited the same districts during 

 the following season, where thej' are referred to late Tertiary or Pliocene 

 age. 



22 GS 



