4 CONTENTS, 



Chapter IV.— The Lower Huronian series— Coiitiiuieil. 



Seetiou 2.— The Mansfield slate— Continued. Page. 



KelatioDS of Maustield slate to adjacent formations 63 



Kelations to iiitrn.sives 63 



Relations to volcanits 6 1 



Strnt'ture of the Mansfield area - 04 



Thicknes.s 64 



Ore deposits - 65 



General descrii)tion of Maustield mine deposit 66 



Relations to surrounding beds 68 



Composition of ore 68 



Microscopical character of the ores and associated chert bauds 69 



Origin of the ore deposits 70 



Conditious favorable for ore concentration '... -. 72 



Exploration . . 73 



Section 3. — The Hemlock formation - - 73 



Distribution, exposures, and topography 73 



Thickness 74 



Relations to adjacent formittions 75 



Relations to intrusives 77 



Volcanic origin - 78 



Classi fication 79 



Acid volcanics - 80 



Acid lavas 80 



Rhyolite-porphyry 81 



Texture - 83 



Aporhyolite-porphyry -- 87 



Schistose acid lavas 87 



Acid pyroclastics 94 



Basic volcanics 95 



Basic lavas - 95 



General characters 95 



Nomenclature - - - 95 



Metabasalts ,- 98 



Nonporphy ritic metabasalts 98 



Petrographical characters 98 



Chemical composition 103 



Porphyritic metabasalt 103 



Petrographical characters -• 104 



Chemical composition - 105 



Variolitic metabasalts 108 



Ellipsoidal structure -.. 112 



Origin of ellipsoidal structure 118 



Amygdaloidal structure 124 



Flattening of amygdaloidal cavities 126 



Alteration of the basalts 126 



Description of some phases of alteration 127 



Pyroclastics 135 



Eruptive breccia 135 



Volcauic sedimentary rocks , 136 



Coarse tuffs 137 



Fine tutfs or ash (dust) beds 142 



Relations of tnffs and ash (dust) beds 143 



Volcanic conglomerates 143 



Schistose pyroclastics 145 



