1018 THE GEOLOGY OF MINNESOTA. 



PLATE VI. 



Page. 

 Mass of sideryte, embracing angular and irregular blocks of silicified obsidian, or 



flint. Rock No. 312, natural size 312 and 994 



No. 1. Sedimentary; mainly siderite, but mingled with much flint, like Nos. 2 and 3, in fine clastic grains. 



No. 2. Flint, or apobsidian, in larger angular masses. 



No. 3. Same as No. 2, but coarser, and weathering rough; distinctly vesicular. 



The rock is essentially a breccia of apobsidian, cemented and charged with siderite. This 

 mineral not only forms the matrix, but also fills the vesicular cavities of the 

 apobsidian. 



