MINERALOGY AND PETROLOGY. 995 



Additional facts.] 



1310, 1311). This siderite is continuously coated with a rusty film from the oxida- 

 tion of the carbonate of iron. The flinty inclusions are not thus coated. It shows 

 a sedimentary structure. Hence it was formed under water, either oceanic or of 

 some local lake, probably the latter. Into that water were carried pieces of volcanic 

 glass from the adjacent land surfaces. .These flint pieces are not taconitic. It was 

 found afterward that a similar breccia-conglomerate is rather widespread, and some- 

 times becomes so fine that its constituent inclusions are globular grains no larger 

 than those of taconyte (Nos. S1SW, 436H). 



3. The carbonate matrix is replaced by one of chalcedonic quartz (No. 436H), 

 and in such form this conglomerate composes thick horizontal beds in the Animikie,* 

 indicating that the siliceous matrix was formed in larger bodies of water, probably 

 in the ocean from which was precipitated chemical silica. In other places the quartz 

 matrix is mingled with the carbonate matrix. In the carbonate matrix is sometimes 

 a thin layer of fine volcanic tuff similar to No. 307 but grayish green instead of 

 black. This difference of color is attributable to its having been deposited in water 

 instead of on a land surface. 



4. This fragmental phase of the Animikie varies still further. It is composed 

 sometimes mostly of rounded quartz grains (No. 1322)- rather coarse; and in this 

 rock are but few grains or pebbles of devitrified glass (figure 9, plate II). Such rock 

 must have been formed largely from the Archean adjacent, but in part from erosion 

 of silicified igneous rocks (of the Animikie ?). It demonstrates that, earlier than 

 the fragmental parts of the Animikie, which include all globular taconyte, was 

 a mass of glassy igneous rocks, and that that earlier rock was probably the same 

 that supplied flint fragments to the siderite (No. 312) illustrated by plate VI. 



5. There are all degrees of proportional gradation between rocks Nos. 1322 and 

 1319, one containing mostly fragmental quartz from the Archean, and the other 

 mostly fragmental devitrified pebbles from silicified rhyolytes. 



6. In No. 1319, as well as in many other taconitic rocks from the vicinity of 

 Gunflint lake, are all degrees of proportional gradation between complete silicification 

 of the component globules and an almost amorphous glassy condition. The same 

 occurs generally in the taconyte of the Mesabi range, but not always in the same rock 

 mass. This shows that the source of supply of these original globules was originally 

 partly silicified and partly not silicified, and hence it is an indication that such may 

 be the case widely on the Mesabi range. These globules are of the same size, originally 

 of the same nature, and since incorporation in the rock have been under the same 

 conditions. If they differ now, they must have differed when they were introduced 

 into this rock, by the same reasoning as shows a difference of origin of the grains of 



*H. V. WINCHELL. Seventeenth Annual Report, p. 104. 



