INDEX. 



lizing out of molten magmas, C9 ; erup- 

 tive with silica, 68 ; films of surrounding 

 magnetite and garnet, 48 ; forming a dike, 

 30 ; secondary origin of, 36, 48. 



Hematite, "soft," at Jackson Mine, 32; 

 composition of, 51 ; conditions of foniia- 

 tion, 50; not bog-iron ore, 51 ; origin of, 

 16, 20, 25, 70; origin and characters of, 

 49-52 ; probable deposit near Lake Ange- 

 line Mine, 50. 



Hodge, James T., origin of copper and sil- 

 ver veins, 84. 



Home Mine, sandstone and ore at, 32 ; jas- 

 per dikes at, 32. 



Hornblende, alteration of, 114, 117, 120; 

 alteration product after augite, 37, 39, 42, 

 46 ; altered to chlorite and biotite, 56 ; 

 foreign in audcsite, 119 ; secondary origin 

 of, 36, 37 ; pseudomorphs after fragments 

 of, 38. 



Houghton, D., his views in regard to the 

 geology of the copper region, 77-81 ; 

 geology of iron district, 2, 3 ; geology of 

 Presque Isle, 2, 3, 62 ; variability of his 

 views, 108. 



Houshton, Michigan, conglomerate west of, 

 121. 



Hubbard, B., on the geological structure of 

 Keweenaw Point, 82 ; on the geology of 

 the iron district, 3, 4 ; on metamorphism, 

 5 ; on the Porcupine Mountains, 83, 84 ; 

 traps eruptive in sandstone, 82. 



Humboldt, Michigan, granite intrusive in 

 schist, 57 ; the rocks and ore of, 35, 36. 



Hungarian Falls, sandstone at, 113. 



Hunt, T. S., his erroneous observations in 

 Eastern Massachusetts, 72 ; his views in 

 regard to the copper region of Lake Supe- 

 rior, 93, 94, 99, 105 ; hematite a proof 

 of presence of organic matter, 14 ; his 

 views in regard to the iron region, 13, 

 19, 24 ; indefiniteness of his statements, 

 109 ; objects to decomposition theory 

 of origin of "soft hematites," 51; not 

 personally acquainted with the iron dis- 

 trict, 40 ; on the Presque Isle serpen- 

 tine, 62 ; on the sedimentary origin of 

 greenstones, 25. 



Huronian and Laurentian in iron region, 

 26. 



Hm-onian, conformable with Laurentian, 25; 

 contact with Laoi'entian not observed, 24, 

 25 ; according to Hunt, equivalent of the 

 Cambrian, 13; eruptive granite in, 55; 

 including rocks of two distinct ages, 31 ; 

 relation to Laurentian, 70, 71 ; younger 

 than the Laurentian, 22, 24. 



Huronian age, according to Bigsby, distinct 

 from the Cambrian, 14 ; of iron district. 



13, 14, 15, 17; of Presque Isle serpen- 

 tine, 62, 64. 

 Huronian rocks in iron region, 24 ; ancon- 

 formable with the Laurentian, 22. 



Indians, the customs of, 92 ; use of copper 

 by, 92. 



Iron age of geology, 9. 



Iron district, country rocks of, 28, 29 ; 

 eruptive rocks of, 46 ; geological struc- 

 ture of, 75 ; historical discussion of, 1- 

 26; Huronian age of, 13, 14, 15, 17; 

 jasper and iron ore of, 28-36 ; not visited 

 by C. T. Jackson, 6; object of the in- 

 vestigation of, 27 ; sea-beaches in, 31 ; 

 summary of geological views on, 26, 27. 



Iron ore, breaking through schist, 31 ; chem- 

 ical and metallurgical objections to erup- 

 tive crigin of, 68, 69 ; has the color and 

 streak of hematite, but is magnetic, 33 ; 

 cutting schist, 31 ; a deep-sea deposit, 25; 

 its deposition considered due to the pres- 

 ence of organic matter, 14 ; dikes of, 30, 

 32; difficulty in mining, 31; history of 

 its discovery, 3. 4, 18 ; eruptive charac- 

 ters of, 32 ; eruptive origin of, 27, 67 ; 

 form of eruption, 68 ; form of masses of, 

 29; Huronian age of, 14, 15, 17; inter- 

 laminations of, with jasper, 29 ; igneous 

 origin of, 7, 9, 10 ; not interstratified 

 with schists, 67 ; intrusive character of, 

 35 ; at Ishpeming, 30, 31 ; junction of, 

 with chlorite schist, 30 ; the Laurentian 

 age of, 13 ; limits between it and the jas- 

 per, 33 ; magnetic in contact with dikes, 

 43 ; magnetic state induced by secondary 

 causes, 34 ; manner of intrusion, 35 ; in 

 Marquette district, 6, 7, 14, 15; in Me- 

 nomonee region, 3, 6, 7 ; in metamorphic 

 rocks, 7; originally magnetite, 17; mi- 

 croscopic study of, intrusive in quartzite, 

 33 ; at Negaunee, 31, 32 ; in octahedrons, 

 33 ; origin of, 12, 13 ; its origin a matter 

 of economic importance, 29 ; the oriorinal 

 state of, 32, 34; plasticity of, 21,^67; 

 according to Rivot, of Potsdam age, 91 ; 

 proof of its intrusive character, 67 ; its 

 relations to the country rock, 30-36; re- 

 lation to schist, 31 ; secondary deposition 

 of, 33 ; sedimentary origin of, maintained 

 bv various geolosists, 6, 13, 14, 15, 17, 18, 

 20, 21, 22, 24,^25, 27 ; not in veins, 19, 

 29 ; wedge-shaped masses of, 31, 33. 



Iron, oxides of, their action in reduction of 

 the copper, 90, 92, 93, 97 ; precipitation 

 of copper by, 130. 



Iron, the protoxide of, an agent in the depo- 

 sition of copper in veins, 90. 



Iron, titaniferous, decomposition of, 48. 



