MUSEUM OF COMPArvATlVE ZOOLOGY. 



33 



It would seem that a microscopic examination of the banded jasper 

 and ore should give us some facts bearing upon the question. A sec- 

 tion was made of a finely-bauded jasper (13G), taken near the Lake 

 Superior mine. Under a lens this shows a line contorted banding. 

 Microscopically this section is composed of a fine granular aggregate of 

 quartz and liematite, and a more coarsely crystallized portion made up 

 of octahedrons of magnetite or martite, and of quartz of secondary origin. 

 The quartz in the first part is largely tilled with minute globules and 

 grains of ore, which also occurs in irregular masses and in octahedrons. 

 The quartz associated with the more coarsely crystallized portion is 

 water clear, and shows the usual fibrous gramdar polarization of second- 

 ary quartz. Wherever the iron is in a distinguishable crystalline form 

 it is in octahedrons. The color and streak of the iron in the hand 

 specimen are those of hematite, but tlie povvtler is found to be nuignetic. 

 No. 153, from the same locality, has similar characters. The section 

 was taken from the most jaspery portion, and shows much of the fmo 

 aggregation of quartz and hematite. The structure of the quartzose 

 portion is like the devitrification structure of the rhyolites anil fclsites. 

 The section has been repeatedly fissured, and the fissures filled in with 

 secondary deposits of quartz and octahedral crystals of iron. So far as 

 we have observed, the brecciated jasper and ore have had their fractures 

 filled in like manner. No. 271, from pit No. 7, Jackson mine, is of similar 

 character. The jaspery portion is finely banded, and shows an apparent 

 fluidal structure. We are inclined to regard the structure as fluidal, 

 but in a rock so deeply colored it is difficult to make satisfactory exam- 

 inations. This is the only section that shows anything like a well- 

 defined limit between the jasper and ore bands, under the microscope, as 

 pointed out by Dr. Wichmann.''^ The powder of the two last-desci'ibed 

 specimens is feebly magnetic. No. 202 was from the Pittsburg and 

 Lake Superior mine, Cascade range. This shows ihe intrusion of the 

 iron ore through the quartzite (p. 32). The ore gives the hematite 

 streak, is feebly magnetic, and appears to be in octahedrons. The 

 quartz is m\ich fissured, shownng the effect of heat, and contains micro- 

 lites and fluid and stone inclusions. 



The octahedral form of the iron ore would sustain the view that it 

 was all oi'iginally nuignetite. The difliculty lies in proving the crystals 

 to be primary, and not secondary forms, especially as they arc largely 

 associated with secondary quartz, and also are abundant in the little 

 fissures (minute veins) traversing the jasper. Our microscopic examina- 



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VOL. VII. —KG. 1. 



* Gcol. of Wise, III. 615. 



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