VI CONTENTS. 



CiiAi'TER IV. — The Lower Huuonian series — Continued. 



Section 2. — The Manslield slato — Continued. Page. 



Structure of the Mansfield area 64 



Thickness 64 



Ore deposits 65 



General description of Mansfield mine deposit 66 



Keliitions to surrounding beds 68 



Composition of ore 68 



Microscoiiieal character of the ores and associated chert bands 69 



Origin of the ore deposits 70 



Conditions favorable for ore concentration 72 



Exploration 73 



Section 3. — The Hemlock formation 73 



Distribution, exposures, and topography 73 



Thickness 74 



Gelations to adjacent formations 75 



Relations to intrnsives 77 



Volcanic origin 78 



Classification 79 



Acid volcanics 80 



Acid lavas 80 



Rhy olite-porphy ry 81 



Texture 83 



Aporhyolitc-ijorphy ry 87 



• Schistose acid lavas 87 



Acid pyroclastics 94 



Basic volcanics 95 



Basic lavas .- 95 



General characters 95 



Nomenclature 95 



Metabasnlts 98 



Xonporphyritic metabasalt 98 



Petrograpbical characters 98 



Chemical composition 103 



Porphyritic metabasalt 103 



Petrograpbical characters 104 



Chemical composition 105 



Variolitic metabasalts 108 



Ellipsoidal structure 112 



Origin of ellipsoidal structure 118 



Amygdaloidal structure 124 



Flattening of amygdaloidal cavities 12f) 



Alteration of the basalts 126 



Description of some phases of alteration 127 



Pyroclastics 135 



Eruptive breccia 13.5 



Volcanic sedimentary rocks 136 



Coarse tutts 137 



Fine tutt's or ash (dust) beds . 142 



Relations of tuffs and ash (dust) beds llli 



Volcanic conglomerates 143 



Schistose pyroclastics 145 



The Bone Lake crystalline schists 148 



Distribution 148 



Field evidence of connection with the volcanics 149 



Petrograpbical characters 150 



