IV 



INDEX. 



chusetts, 72 ; jointing of, 37 ; microscopic 

 study of, 38, 39, 118-120; its occur- 

 rence near Marquette not noticed by 

 Julicn, Credner, Brooks, or Roniinger, 38 ; 

 originally identical with modern rhyolites, 

 38, 39 ; its occurrence in the iron region, 

 75 ; its present difference from rhyolite 

 due to alteration and depth, 38, 39 ; its 

 relation to rhyolite, 118, 119. 



Ferrite in pebbles, 113. 



Field evidence, value of, 43, 74. 



Fluid cavities in quartz, 38, 39, 44, 45 ; ly- 

 ing in fissures, 54. 



Fluidal structure, 28 ; in felsite, or rhyolite, 

 88, 118, 119; segregation of minerals 

 along lines of, 38 ; in granite parallel to 

 contact planes, 57. 



Foliation not necessarily coincident with 

 stratification, 25 ; no proof of stratifica- 

 tion, 72 ; of granite a fluidal structure, 

 57 ; relation to pressure, 53. 



Forest Copper Mine, sandstone at, 100. 



Fossils, absence of, explained by thermal 

 water action, 116. 



Foster, J. W., description of the iron region, 

 15, 16 ; on age of sandstone, 84 ; advo- 

 cates the sedimentary origin of the iron 

 ores, 6 ; on the origin of the " soft hema- 

 tite," 16. 



Foster, J. W., and Whitney, J. D., azoic 

 age of district, 26 ; on the Carp River 

 sandstone, 60 ; definition of the Azoic 

 System, 7, 8, 9 ; on the formation of 

 amygdaloid and conglomerate, 86, 87, 88 ; 

 the results of their geological work sus- 

 tained, 131 ; the iron age in geology, 9 ; 

 on the metamorphic rocks, 8, 9 ; on the 

 mode of occurrence of copper, 86 ; objec- 

 tions to their view of the relations of the 

 copper-bearing rocks and Eastern sand- 

 stone in the vicinity of Torch Lake, 122 ; 

 objection to their view of the structure of 

 Presque Isle, 62, 63, 76 ; on the occur- 

 rence of the iron ore, 7 ; on the geologi- 

 cal age of Eastern sandstone, 122 ; on the 

 geological structure of Keweenaw Point, 

 86-89, 96, 97 ; on sand concretions in shale, 

 121 ; on the origin of the copper veins, 

 88 ; on the Potsdam age of the sandstone, 

 87 ; on the Presque Isle serpentine, 62 ; 

 on the relations of the trap and sandstone, 

 111 ; their Report on the Geology of the 

 Lake Superior Land District, 9; Silurian 

 age of the sandstone, b6 ; their theory of 

 the origin of the iron ore sustained, 68 ; 

 the traps a succession of lava flows, 86. 



G.\RNET, altered forms of, 53 ; columnar 

 structure of, in schist, 48 ; dodecahedral 



crystals in diorite, 47 ; films of hematite 

 surrounding, 48 ; inclusions in, 47 ; in 

 dioi'ite, 46 ; secondary origin of, 47. 



Garnetiferous rock, microscopic study of, 48. 



Geikie, A., microscopic distinction between 

 lava flows and intrusive masses, 111 ; ex- 

 ceptions thereto in the Lake Superior dis- 

 trict, 111. 



Geological theories of copper district, 76- 

 i09 ; of iron district, 1-27. 



GeoloKv of Lake Superior, bibliography of, 

 133-157. 



Glass, an unstable rock form, 125. 



Gneiss, cut by diorite aud diabase, 58 ; mi- 

 croscopic study of, from various mines, 

 57, 58 ; modified in contact with granite, 

 55 ; origin of, 57 ; sedimentary origin of, 

 26, 58. 



Granite, contact with a garnetiferous actino- 

 lite rock, 52, 53 ; cutting diabase, 39 ; cut- 

 ting quartzite, 55 ; cutting schist, 52, 57 ; 

 eruptive in Huronian, 55 ; eruptive, of 

 Eastern Massachusetts, 71, 72 ; eruptive 

 origin of, 11, 24 ; foliation of, 52, 53 ; 

 Huronian age of, 24, 25; igneous origin 

 of, 10; intiusive in schists, 55; Lauren- 

 tian age of, 14, 15, 19, 24, 25, 52; mi- 

 croscopic study of, from various localities, 

 53-57 ; passage of foliated into non-foli- 

 ated forms, 57 ; passage into greenstone, 

 2 ; passage into sei'pentine, 2 ; of Picnic 

 Point, 39 ; Potsdam age of, 91 ; presence 

 of, in iron region, 75 ; red, intrusive in 

 gray gneiss, 55 ; of Republic Mountain, 

 52-55 ; section of intrusive, in gneiss, 57 ; 

 sedimentary origin of, 14, 25 ; younger 

 than Huronian rocks, 53. 



Granite Point, relations of granite and sand- 

 stone, 1, 2. 



Granite porphyry, microscopic study of, 

 114, 117. 



Gra])hic granite, alteration structure resem- 

 bling, 114, 115, 117, 119, 120. 



Gray, A. B., report on iron district, 4. 



Greenstone, a metamorphosed sedimentary 

 rock, 80 ; defined, 128 ; of Keweenaw 

 Point, 111. 



Greisen, eruptive origin of, 54, 55 ; a modi- 

 fication of granite, 55 ; of Republic Moun- 

 tain, 44, 53-55. 



Groundmass, alteration of, 43. 



Hall, J.\mes, maintains the Potsdam age 



of the sandstone, 93. 

 Hancock ^line, "ash bed " not found at, 



112, 113. 

 Hancock, Michigan, conglomerates at, 121, 

 Hematite, a decomposition product, 59 ; al> 



teration product of magnetite, 39 ; crystal. 



