PLATE XII. 



A, Photomicrograph showing the granules in the Gunflint formation. These granules, which 

 originally consisted of a green to brownish-green hydrated ferrous silicate, may, after alteration, 

 consist of limonite, hematite, magnetite, ferrugi;ious carbonate, silica, actinolite, and griinerite, in 

 various combinations. Limonite and silica occur very commonly. In this slide the granules consist 

 of hematite and silica. The spherulitio character of the siliceous matrix is well shown by the black 

 crosses. (Slide 29446, 21 diameters, with analyzer, p. 380.) * 



B, Photomicrograph showing the details in a limonite silica granule. The limonite occurs at 

 the centers and around the peripheries of small spherulitic or granular areas of silica. In the upper 

 right-hand quadrant an area with well-defined agate structure is distinctly shown. (Slide 7004, 80 

 diameters, without analyzer, p. 383. ) 



