254 Geological Excursion into Central Wisconsin — Hall. 



conspicuous exposure of the gabbro. The rocks are not high, they 

 are glaciated and river-worn at the surface, and have been quarried 

 sufficiently to enable the collector to secure very fresh material. 

 Exposures in several localities to the south and east are said to 

 occur. In texture the rock is medium to coarse grained, and its 

 color is dark owing to the large proportion, about two-thirds, 

 which the basic minerals, pyroxene, hornblende and biotite make 

 of its bulk. Pyrite and chalcopyrite appear as accessories scattered 

 through the freshest part of the rock. Many joints appear in the 

 rock, and along them a green alterations product is spread like a 

 thin film. Occasionally this is stained with ferric oxide. The 

 joints can be grouped into two systems, the one extending N. lOS 

 E. magnetic, and the other N. 45*^ W. 



Coursing through this gabbro in every direction are veins of a 

 lighter color and a much finer texture. They vary in width from 

 a fraction of an inch to more than two and one-half feet. As a 

 rule these veins are light colored and finely textured, although in 

 a few places a pegmatitic structure and texture can be seen. A 

 few occurrences of a darker colored veinstone appear. Near these 

 darker colored bands are many gneissic modifications of the rock, 

 as there are at the East Saint Cloud quarries in Minnesota. These 

 gneissic bands have no definite width or position; they appear and 

 disappear in all directions and are frequently interrupted, but no- 

 where are they of great extent. So it is difficult to regard them 

 as resulting from pressure applied steadily in any one direction; 

 they may therefore be regarded not as flowage lines, but either as 

 due to a fiowage structure which existed in places in an original 

 eruptive rock, or they may represent the utter demolition of the 

 original crystalline structure and its replacement by the present 

 one. Inclusions quite different in outward appearance from the 

 veinstuff and gneissic bands are also seen; they are of a dark color 

 and biotite in large folia gives a porphyritic appearance to them. 

 No lithologic significance is attached to these inclusions more than 

 that they represent areas in which a larger proportion of basic con- 

 stituents has promoted a more extensive alteration of the primary 

 constituents of the rock. In structural features these masses remind 

 one of the augitediorite* bodies of Richmond and Little Falls, Minn. 

 *This name was given in 1887 by Streng and Kloos, Neues Jahrbuch 

 fur Mineralogie, n. s. w., 1877, pages 117, et. seq. The more recent studies 

 of the writer show these rocks to be hypersthene bearing gabbros. Compare 

 forthcoming bulletin U. S. geol. survey. 



