256 Geological Excursion into Central Wisconsin — Hall. 



Rhinelander, showing the following result: 



Metallic iron, 59.31 per cent. 



Phosphorus, 0.017 per cent. 



It is very probable that these are segregations of iron oxide 

 precisely similar to those occurring in the gabbros of so many 

 other localities in the Lake Superior basin, notably in northeastern 

 Minnesota, along the so-called Mesabi range. 



The veins and other modifications which have been mentioned 

 as occurring numerously are of varying extent, and are from some- 

 what basic to highly acidic in their chemical composition. 



The microscopic characters of these incidental phases of the 

 Rhinelander gabbro are not complex. A large proportion of 

 quartz, in small, brightly polarizing grains, arranged as a sort of 

 mosaic work, characterizes them all, but there are some larger areas 

 of this mineral filled with liquid inclusions, needles of rutile, and 

 minute crystals of apatite, bearing every evidence of being of 

 original development. The feldspar is partly of the ancient order 

 and partly secondary. That which seems to be original is consid- 

 erably corroded on its borders, giving place there to quartz, other 

 feldspars and epidote. Many of these older individuals show a dis- 

 tinct zonal structure in their central portions. The general acid- 

 ification which these rocks seem to have undergone would suggest 

 the correctness of the explanation Hopf ner has given of this phe- 

 nomenon: — ''Probable that the feldspathic material, which^ in 

 many cases, built up the small isolated secondary grains, when in 

 contact with these older areas, enlarged them to their present new 

 dimensions." Several varieties of feldspar abound ; microclinic 

 and albitic types of low extinction are most abundant. The epi- 

 dote is thickly strewn in many sections, but it is plainly secondary; 

 it lies in small granules, seldom rising to the dignity of individ- 

 uals, which can be measured by optical methods. Biotite and 

 hornblende are, ia the more acidic veins, very sparse, but in the 

 schistose bands and dark-colored inclusions they are comparatively 

 abundant, and are distributed in nearly equal proportions. They 

 carry needles of rutile and crystals of apatite, as well as the quartz. 

 The hornblende frequently twins, and both minerals are very free 

 from inclusions of all kinds. 



At Wausau, where the next masses of gabbro were seen, the 

 conditions of occurrence seemed to be identical with those at 

 Rhinelander; large bosses of a dark-colored rock, anastomosed with 



