Part I.—THE WESTERN PART OF THE DISTRICT DEPARTMENT OF THE INTERIOR By J. MORGAN CLEMENS MONOGRAPHS OF THE CONTENTS. United States Geological Survey LETTER OF TRANSMITTAL......................................................4 VOLUME XXXVI INTRODUCTION, BY CHARLES RICHARD VAN HISE ..................5 OUTLINE OF MONOGRAPH. ...................................................9 PART I.—THE WESTERN PART OF THE CRYSTAL FALLS DISTRICT, BY J. MORGAN CLEMENTS. CHAPTER I.—INTRODUCTION..............................................13 Previous work in the district................................... 14 Mode of work. ........................................................18 Magnetic observations. ..............................................18 CHAPTER II.—GEOGRAPHICAL LIMITS, STRUCTURE AND WASHINGTON STRATIGRAPHY, AND PHYSIOGRAPHY...................................19 GOVERNMENT PRINTING OFFICE Geographical limits. ...............................................19 1899 Structure and stratigraphy. ....................................19 UNITED STATES GEOLOGICAL SURVEY Physiography.........................................................21 CHARLES B. WALCOTT, DIRECTOR Topography................................................................21 Drainage. ...................................................................21 Timber and soil...........................................................24 THE CHAPTER III.—THE ARCHEAN.............................................25 CRYSTAL FALLS IRON-BEARING DISTRICT OF Distribution, exposures, and topography............... 25 Relations to overlying formations. .........................25 MICHIGAN Petrographicai characters......................................26 Biotite-granite (granitite).............................................26 BY Gneissoid biotite-granite, border facies of granite......27 J. MORGAN CLEMENTS and HENRY LLOYD SMYTH Acid dikes in Archean.................................................28 WITH Basic dikes in Archean...............................................28 A CHAPTER ON THE STURGEON RIVER TONGUE Schistose dikes. .....................................................28 BY Massive dikes.........................................................29 WILLIAM SHIRLEY BAYLEY Résumé. ................................................................29 AND CHAPTER IV.—THE LOWER HURONIAN SERIES....................30 AN INTRODUCTION Section 1.—The Randville dolomite.............................30 BY CHARLES RICHARD VAN HISE Distribution, exposures, and topography............... 30 Petrographical characters......................................30 Relations to underlying and overlying formations. 31 Thickness. .............................................................31 Section 2.—The Mansfield slate. .................................31 Distributions, exposures, and topography............. 31 Possible continuation of the Mansfield slate. .............32 Petrographical characters......................................32 Graywacke. ................................................................32 Clay slate and phyllite. ...............................................33 Origin of clay slate and phyllite...............................33 Present composition necessarily different from that WASHINGTON of rock from which derived. ....................................33 GOVERNMENT PRINTING OFFICE Analysis of Mansfield slate.....................................33 1899 Comments on analysis,..........................................34 Comparison of analysis of Mansfield clay slate with analyses of clays....................................................34 Comparison of analysis of Mansfield clay slate with analyses of other clay slates. .................................34 Siderite-slate, chert, ferruginous chert, and iron ores. 34 Monographs of the USGS Vol. XXXVI – Part I – Page 1 of 133 Relations of siderite-slate, ferruginous chert, and ore CHAPTER V.—THE UPPER HURONIAN SERIES. ....................73 bodies to clay slates. ..................................................35 Distribution, exposures, and topography............... 73 Relations of Mansfield slate to adjacent Magnetic lines. ...........................................................74 formations. .............................................................35 Thickness. .............................................................74 Relations to intrusives. ...............................................35 Folding...................................................................74 Relations to volcanics.................................................35 Crystal Falls syncline. ................................................74 Structure of the Mansfield area..............................36 Time of folding of the Upper Huronian. ......................75 Thickness...............................................................36 Relations to other series........................................ 76 Ore deposits...........................................................36 Relations to intrusives. ..........................................76 General description of Mansfield mine deposit...........37 Correlation. ............................................................76 Relations to surrounding beds....................................37 Petrographical characters......................................77 Composition of ore. ....................................................38 Microscopical character of the ores and associated Sedimentary rocks. ....................................................77 Microscopical description of certain of the chert bands.................................................................38 sedimentaries.........................................................78 Origin of the ore deposits. ..........................................38 Igneous rocks.............................................................81 Conditions favorable for ore concentration.................39 Exploration. ................................................................39 Ore deposits. .........................................................81 History of opening of the district.................................81 Section 3.—The Hemlock formation............................ 39 Distribution. ................................................................81 Distribution, exposures, and topography. ..............40 Western half of sec. 34, T. 46 N., R. 33 W.............81 Thickness...............................................................40 Sec. 20, T. 45 N., R. 33 W. ....................................81 Relations to adjacent formations. ..........................40 The Amasa area.....................................................82 Relations to intrusives............................................41 The Crystal Falls area. ...........................................82 Volcanic origin........................................................41 Character of the ore. ..................................................84 Classification..........................................................42 Relations to adjacent rocks. .......................................84 Acid volcanics. .......................................................42 Origin. ........................................................................85 Acid lavas...................................................................42 Size of the ore bodies. ...............................................85 Rhyolite-porphyry. ..................................................43 Methods of mining......................................................85 Texture of the porphyries....................................43 Prospecting. ...............................................................86 Aporhyolite-porphyry. .............................................45 Production of ore from the crystal falls area...............86 Schistose acid lavas...............................................45 CHAPTER VI.—THE INTRUSIVES. ........................................86 Acid pyroclastics.........................................................48 Order of treatment. ................................................87 Basic volcanics. .....................................................48 Age of the intrusives..............................................87 Basic lavas. ................................................................48 Relations of folding and the distribution of the General characters.................................................48 intrusives....................................................................87 Nomenclature. ........................................................49 Metabasalts. ...........................................................50 Section I.—Unrelated intrusives...................................88 Nonporphyritic metabasalt. .................................50 Classification. ........................................................88 Petrographical characters...............................50 Acid intrusives. ......................................................88 Chemical composition.....................................52 Geographical distribution and exposures of granites. 88 Porphyritic metabasalt. .......................................52 Biotite-granite.............................................................88 Petrographical characters...............................52 Micropegmatites.....................................................89 Chemical composition.....................................53 Muscovite-biotite-granite.