2IO Bidleiin of Laboratones of Denison University. [v..i. xii 



ous iron oxide. Next to quartz, pyrite is the most abundant 

 mineral. It frequently occurs as fresh grains and crystals ; 

 more often as partially and entirely oxidized in the form of iron 

 oxide. In the latter case the original pyrite may be altered to 

 a limonite pseudomorph, but generally the change has been 

 rapid and a cavity preserving the outline of the pyrite, only 

 slightly stained or partially filled with the iron oxide, forms the 

 only evidence of the former presence of the pyrite. The min- 

 eral, both in the fresh and in the completely oxidized or altered 

 condition, is often present in the same thin section. The stain 

 of yellow iron oxide derived from the oxidation of the pyrite 

 discolors the section for some distance around and away from 

 the position of the original sulphide mineral. The stain- 

 ing extends the farthest along the sutures between the quartz- 

 grains and the fracture-lines, which are present at times in the 

 grains. 



Microscopic study shows quite plainly the relations of the 

 ocher to the quartzite and its mode of occurrence in the rock. 

 After a careful study of a number of thin sections under the 

 microscope of the ocher-stained rock from different parts of the 

 belt, my results accord so closely with Hayes's description that 

 I quote him in full. He says : ' 



"When the transition-rock is examined under a microscope, the character of 

 the transition can be seen even more clearly. The more compact portions, which 

 are only slightly stained with iron, are seen to be composed of a transparent 

 ground-mass, threaded with minute cavities, which penetrate the rock in all direc- 

 tions and contain a fine dendritic growth of iron oxide. The latter occurs only 

 rarely in isolated grains, but generally in clusters of minute grains or fibers, at- 

 tached to each other and branching irregularly from a central stem. They have 

 no trace of crystal form. Passing toward the ore-body, these minute passages 

 become larger and increase in frequency, until only a finely branching siliceous 

 skeleton remains, the greater part of the rock having been replaced by the iron 

 oxide. Under polarized light, the transparent ground-mass is broken up into an 

 aggregate of small quartz-grains, penetrated in all directions by the iron oxide. 

 The latter does not lie between the individual grains, but passes through them, 

 as though the ground-mass were quite homogeneous. The process of replacement 

 is never complete ; for all the ocher contains more or less sand. When this is 

 washed clean from the iron oxide, it is found to differ from ordinary sand-grains 



' Trans., IQOI, xxx., 416. 



