46 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
LIMESTONE. 
The first limestone noticed in my journey from Boise eastward was 
at Martin post-office, where there are ledges of this rock, of a bluish 
color, changing locally to nearly pure white and containing crinoid 
stems. The strike is southwest and the dip 15° northwestward. 
About 3 miles to the west of the locality just referred to and on the 
west side of the valley of Lava Creek a conspicuous ridge of bluish 
limestone, bearing about northeast and southwest, was in view, but was 
not visited. In the valkyy of Little Lost River, near Howe, other out- 
crops of bluish limestone occur. 
The age of the limestone and of the associated and apparently far 
more abundant quartzite is not known, but judging from their loca- 
tion and their relation to terranes occurring farther north, in the 
vicinity of Hailey, Challis, etc., examined by G. H. Eldridge/'and from 
their resemblance to other formations of known age in southeastern 
Idaho and the adjacent portion of Utah, it is probably Paleozoic. It 
seems likely that future studies will show that they range in age from 
the Silurian to the Carboniferous. 
ROCKS OF SOUTHEASTERN IDAHO. 
The geology of the extreme southeastern portion of Idaho, lying 
east and southeast of Snake River, was studied by the geologists of 
the "Hayden survey" in 1871 and subsequent years, and is repre- 
sented on a "geological map of portions of W}^oming, Idaho, and 
Utah" which accompanies the twelfth annual report of that survey. 6 
The portion of Idaho referred to has not been examined by me, but 
judging from the map mentioned above and the information contained 
in the itinerary reports by F. M. Endlich, F. H. Bradley, A. C. Peale, 
and Orestes St. John, published in the sixth and eleventh annual 
reports of the survey just referred to, it has a close resemblance in 
the nature of its rocks, structure, topography, etc. , to the region to 
the west of the Snake River Plains, known as the Lost River country. 
The rocks forming the prominent mountains to the east of Snake 
River are largely limestone and quartzite of Silurian and Carbonif- 
erous age, while the valleys, beginning on the east, near Soda Springs, 
and extending northwestward to the Snake River Plains, are mostly 
floored with basalt. In addition to basalt, the valleys contain deep 
accumulations of debris swept down from the neighboring uplands 
and deposited as alluvial fans. 
LEADING FEATURES OF THE GEOLOGICAL STRUCTURE. 
The general trend of the mountains in southeastern Idaho, as 
indicated on the maps published by the Hayden survey referred t( 
a Sixteenth Ann. Kept. U. S. Geol. Survey, 1895, pp. 226-230. 
&U. S. Geological and Geographical Survey of the Territories; a report of progress of the explor 
tions in Wyoming and Idaho for the year 1878, Washington, 1883. 
