CONTENTS. 



Page. 



CHAPTER I. General topography and geology 625 



Geographical position 625 



Snake River Valley .. _ 625 



Literature , 626 



Topography 628 



Geological history 630 



Pre-Tertiary 630 



The Payette formation 632 



Early Neocene volcanic activity 634 



Post-Payette erosion ... 635 



Post-Payette orographic disturbances 635 



Late Neocene basalts ... 636 



Post-basaltic erosion.. ,. 636 



Resume of geological events in the lower Snake River Basin 637 



CHAPTER II. The ore deposits in general 638 



General features 638 



Alteration of the country rock _ ._. 638 



Silicification ... 645 



Structure of the veins. ... 647 



Mineral deposits of post-Neocene age . _ _ 648 



CHAPTER III. The Idaho Basin 651 



Geographical position ... 651 



Discovery and history ,-. 651 



Production . 652 



Topography.. 656 



Grades of the water courses . .' 657 



Tertiary and Pleistocene formations 657 



Lower Moore Creek Valley .... . 657 



Configuration of valley . 657 



The basalt flow 658 



The present stream gravels .... 658 



The gravels below the basalt 658 



High gravels . 659 



Upper Moore Creek Valley. . 659 



Configuration of valley ... 659 



The present stream gravels 660 



Bench gravels _. . 660 



High gravels 662 



Lake beds 665 



Gold in the lake beds 668 



Olivine basalt (dolerite) _. - 669 



The valley of Grimes Creek 669 



Configuration 669 



619 



