400 THE MARQUETTE lEON-BEARING DISTRICT. 



body at almost au}' place. The ore bodies grade above and at the sides 

 into the jasper in a variable manner. As a result of the combination of 

 these imcertain factors, most of tlie ore bodies liave extraordinarily in-eg- 

 idar and cmious forms when examined in detail, although in general shape 

 they conform to the above descriptions. 



While these different classes of ore bodies have the distinctive features 

 indicated above, they have important features in common. They are con- 

 fined to the iron-bearing formation. They occur upon impervious basements 

 in pitching troughs. The im]jervious basement may be a sedimentary or 

 igneous rock, or a combination of the two. Where the ore deposits are of con- 

 siderable size the plication and brecciation of the chert and jasper are usual 

 phenomena. (Pis. XX-XXIII and XXV-XXVII.) Frequently this shat- 

 tering was concomitant with the folding into troughs or with the intrusion of 

 the igneous rocks. When the passage of the ore bodies into the chert or 

 jasper is examined in detail, it is found that a siliceous band, if followed 

 toward the oi'e, instead of remaining solid, becomes porous and frequently 

 contains considerable cavities. These places in the transition zone are lined 

 with ore. In passing toward the ore deposit more and more of the silica is 

 found to have been removed, and the ore has replaced it to a coiTesponding 

 degree. (PI. XXIII, fig. 1.) An examination at many localities shows this 

 transition from the banded ore and jasper to take place as a consequence of 

 the removal of the silica and the substitution of iron oxide. In such instances 

 the fine-grained part of the ore is often that of the original rock, while 

 the coarser crystalline material is a secondary infiltration. (Pis. XXIII 

 and XXVI.) It is not infrequently the case, however, that the ore deposits 

 aljruptly tenmnate along a joint crack or fracture. (PI. XXVIII, fig. 3.) 



ORIGIN OF THE ORES. 



Tlie facts given in the foregoing pages in reference to the iron-bearing 

 formation and its origin, combined with the peculiar occurrence of the ores, 

 indicate with certainty the main features of the origin of the ore deposits. 



While the ore deposits of the Lower Marquette series have a greater 

 variety of form and relations than those of the Penokee district,^ it is evident 



' The Penokee iron-bearing series of Michigan and Wiaconsin, by R. D. Irving and C. E. Van 

 Hise: Mon. U. S. Geol. .Survey. Vol. XIX, 1892. pp. 280-290. 



