390 W. S. Bayley — Spotted Rocks from Minnesota. 



QJ 



is very irregular. Frequently a single spot stands alone. Some- 

 times two or more spots are united, and when this is the case 

 one rim encloses the group. Figure 1 represents the shape and 



general appearance of some 

 of these groups as seen on 

 a smooth weathered surface. 

 When the rock bearing 

 such spots is broken it is 

 observed that the bodies 

 producing them are them- 

 selves either lenticular in 

 ®^ ^ shape or spherical. They 



Fig. 1. About one-third actual diameters. possess a SU g ary texture and 



are of a pistachio green color. When moistened with hydro- 

 chloric acid they effervesce with a slight evolution of gas. 

 Like the circles on a weathered surface these spheroidal bodies 

 are also surrounded by a narrow brick-red rim. The rims 

 are here fairly well defined against the substance which they 

 enclose, but on their onter edges gradually shade off into the 

 body of the rock. 



Associated with these spotted rocks, but at a greater distance 

 from the eruptives, are other quartzites which show no spots 

 on a weathered surface, but which contain little concavities 

 with diameters of about the same magnitude as those of the 

 spots mentioned above. On a fresh fracture of these, instead 

 of the green spheroidal bodies, are circular and oval areas,, 

 which reflect the light evenly, as from a smooth cleavage sur- 

 face, and show a silvery luster. When treated with acids they 

 effervesce very briskLy. They are nothing more than -concre- 

 tions of calcium carbonate so often met with in the slates and 

 sandstones of many regions.* 



Under the microscope the body of a specimen of one of 

 these rocks (]N"o. 11,461 of the collection of the Lake Superior 

 Division IT. S. Geological Survey) is seen to be composed of an 

 aggregate of quartz, feldspar and green mica. r The quartz is 

 in rounded grains, which everywhere show secondary enlarge- 

 ment. Much of the feldspar is triclinic. The green mica is 

 slightly pleochroic and seems to have crystallized in position. 

 A few grains of magnetite are scattered through the section, 

 and small areas of calcite are occasionally found enclosing the 

 other constituents. The silvery spots consist of perfectly trans- 

 parent calcite. This encloses all the other constituents, which 

 are the same as those found in the main portion of the section. 

 It polarizes in large areas, although several of these are seen 

 in a single spot. 



* Ct. Tschermak : Lehrbuch der Mineralogie, Auf. II, p. 117. 



