MINERALOGY AND PETROLOGY. 949 



Siderite.] 



surround the included quartzes, due to the high double refraction of the adjacent edges 

 of the siderite, both of which are absent from the siderite globules when quartz is 

 not adjacent. It is owing to their globular composite structure that these rhombs 

 seldom give an interference figure, however cut by the section. 



In some of the best sections of jaspilyte that have been examined (Nos. 903, 907), 

 all the iron ores are present, the magnetite, pyrite and siderite in somewhat coarse 

 crystals, and the hematite in minutely fine particles disseminated amongst the 

 finest quartz. If any distinction as to priority of origin can be drawn from this 

 section, it is in favor of hematite, since it is distinctly embraced in the siderite as 

 nuclei of the fine rhombs of that mineral (No. 1565). The magnetite is in distinct 

 octahedra and the pyrite in cubes. These must be later than the hematite, which 

 is in dust-like scales and powdery aggregates distributed amongst the fine jaspilitic 

 quartz, and appears to be the cotemporary of it in origin and like it in manner of 

 deposit. The hematite and quartz are non-differentiated by independent coarser 

 crystallizations. The pyrite, magnetite and siderite are in large (microscopic) crys- 

 tals and crystalline aggregates. 



As to the origin of this early siderite it is to be attributed to some source which 

 allows for the access of carbonic acid to the iron which was taken up by its forma- 

 tion. If the jaspilytes were formed in the bottom of the Archean ocean by chemical 

 precipitation,* carbonic acid may have been derived from the atmosphere primarily, 

 and secondarily from the ocean. It will be seen, however, below, that it is not 

 improbable that this siderite originated in the manner similar to that which perme- 

 ates the iron-bearing rocks of the Mesabi Iron range, and that those rocks were not 

 the product of oceanic chemical precipitation. 



The mineralogical environments and the petrographic structures of the siderite 

 of the Mesabi Iron range are, in general, so much like those of the siderite of the 

 Vermilion range that it seems necessary, in the light of the detailed examina- 

 tion presented in Part II, to consider them one in origin though differing widely as 

 to date. The greater recentness of the Taconic ores seems to have been favorable 

 for the preservation of some of those bonds of alliance with attendant conditions by 

 which their origin and history can be traced out, but which, in the Archean ores, are 

 so far destroyed that the relations of cause and effect cannot be detected. It is by 

 reason of the microscopical and other examinations of a wide series of iron-bearing 

 rocks of the Mesabi range, extending from Gunflint lake to the Mississippi river, that 

 the writer has been led to regard the origin of those rocks very differently from the 

 view formerly entertained by him, and quite different from the opinions presented 

 by other geologists. At this place, however, it is designed to discuss only the sider- 



*Sullcttn vi of the Minnesota (>'< />t<>-ti/ Purvey. 



