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CONTENTS 
Part I. Physiography and Geology. 
Page. 
Introduction ' 13 
Nature and object of the reconnaissance 13 
Topography 14 
Climate 17 
Vegetation 22 
Fauna 24 
Soil 24 
Streams 25 
Springs 26 
Water power 28 
Irrigation and agriculture 28 
Stock raising 31 
Mining 32 
Towns 32 
Railroads 33 
Highways 33 
Big, Middle, and East buttes 34 
General geology 38 
Previous investigations 38 
Pre-Tertiary formations 39 
Granite 39 
Rhyolite 42 
Quartzite 45 
Limestone 46 
Rocks of southeastern Idaho 46 
Leading features of the geological structure 46 
Early geological history of the Snake River Basin 48 
Tertiary and recent lacustral formations 50 
Payette formation 50 
Idaho formation 51 
Fossils 56 
Summary of lacustral conditions 57 
Economic problems 58 
1 '< >ssible eastward extension of the Tertiary sedimentary beds 59 
Snake River lava 59 
Extent and thickness 59 
Relation to the Columbia River lava 60 
Hypothesis of fissure eruptions 61 
Source of the lava 63 
