GEOLOGICAL EXPLOKATIOXS AND LITEKATURE— 1SS6. 109 



of the carbonates may be detected. Some of tlie magnetite mines appear 

 to be working on the richer portions of the magnetitic schists. 



(6) Some of the sihcifying process went on before the folding of the 

 foi-mation, and some of it afterward; and to tlie hiter period beh)ng the 

 larger bodies of crystalline ore, the crntnbling and shattering of the layers 

 aifording the best conditions for the action of the silicifving waters. 



PuMPELLY, Raphael. Report on the itiining indu.stries of tbe United States 

 (exclusixe of tbe precious luetals), with special investigations into the iron resources 

 of the Republic, etc. Department of the Interior, Census Office, Vol. XV, Washing- 

 ton, ISSO, pages 1-82. With maps and plates. 



In the report of the Tenth Census on the iron ores, Pumpelly gives a 

 brief survey of the IMarquette district and records a number of analyses of 

 its ores. Very little that is new concerning the geology of the district is 

 communicated, as the work is largely a compilation. The ores are stated 

 to be in the Huronian, which is regarded as the upj^er member of the 

 Archean, unconformably reposing upon the schists of the Lain-entian. 



While the iron series occiu's nniformlj- in the lower i)art of the Huronian, its 

 structure and character vary in different places. In the Negaunee district it consists 

 of a lower and an upper series. The lower is made up of tlag ores, siliceous and fer- 

 ruginous schists, and some argillaceous and talcose slates and anthophyllitic schists 

 and beds of diabase. * * * 



The upper series, which is separated from the lower by a bed of diabase and a 

 thinner bed of chloritic and talcose slate, contains the rich ores. It consists of a thick 

 mass of banded iron and "'jasper," the iron ore being pure and the jasper generally 

 colored red. * * * In places subsequent chemical action has removed portions of 

 the jasper, while the space thus formed has been filled with limonite in large quayti- 

 ties, as at the Lake Superior mine. The upper part of this upper series has generally 

 a bed of talcose slate, in which the fissile cleavage is wholly independent of the 

 bedding, and which is impregnated with small octahedra of martite. The ore in 

 contact with this slate has the same structure and is impregnated with similar crys- 

 tals of martite. The upper portion of the upper series consists of beds of rich ore, 

 often granular martite, with talcose schists and very talcose quartzites. (Pp. 7-8.) 



The author notes that in the western part of the Marquette basin the 

 ores are largely magnetic. The basin itself narrows and then widens out 

 to form "the broad Huronian area of the central ])art of the Ujiper Penin- 

 sula," where the slates of the uppermost horizons are tlie predominant 



