4 CONTENTS. 
Chapter III. — Sedimentary formations — Continued. Page. 
Quaternary system 44 
Pleistocene conglomerate 44 
Pleistocene and Recent gravel, sand, and clay 44 
Chapter IV. Igneous rocks 46 
Laccoliths (early Miocene) 46 
Distribution and structure 46 
Petrography 48 
Metamorphism of andesite by the ore-bearing solutions 49 
Effusives (late Miocene) 50 
Distribution and structure 50 
Petrography 50 
Early trachyte 50 
Early tuffaceous rhyolite 51 
Later trachyte 52 
Hornblende andesite breccia and agglomerate 53 
Latest trachyte 53 
Pyroxene andesite agglomerate and breccia 55 
Biotite dacite 55 
Late 'tuffaceous rhyolite 56 
Biotite-hornblende-pyroxene andesite 56 
Chemical and mineral composition of the igneous rocks 57 
Relations of laccoliths and effusives in genesis 61 
Chapter V. Correlation t 62 
Sedimentary rocks 62 
Igneous rocks 65 
Chapter VI. Description of the iron ores 66 
Distribution, exposures, and topography 66 
Geological and structural relations of the ore deposits 66 
Deposits in andesite 67 
Deposits at andesite-limestone contact 67 
Deposits in breccias 69 
Kinds and grades of ore 70 
Size and quantity of ore deposits 73 
Development •. 73 
Chapter VII. Origin of the iron ores 75 
General 75 
Concentration and alteration. 75 
Deposition of ore following laccolith intrusions as igneous after-effects 75 
Introduction of the ore 75 
Source and condition of the ore-bearing solutions 78 
Concentration of ore by weathering prior to eruption of Tertiary lavas 79 
Concentration under the influence of Tertiary lavas 80 
Alterations subsequent to the Tertiary eruptions 82 
Summary of mineral associations of ores in relation to origin 84 
Conclusions as to the origin of the ores , 85 
Bearing of the theory of origin upon further exploration ■ 86 
Chapter VIII. Comparison of Iron Springs iron ores with other iron 
ores 90 
Bull Valley district 90 
Deposits of similar origin in other districts 92 
Structural dissimilarities of Iron Springs deposits to other classes of iron-ore 
deposits 95 
Index '. , . . . , ,...„,, , , T , 99 
