Geological Excursion into Central Wisconsin — Hall. 255 



In mineral constitution this Rhinelander gabbro is hypers- 

 thenic and hornblendic. The feldspar is still well preserved and 

 polarizes strongly, although in places some alteration has taken 

 place. Its optical characters show it to be a labradorite with a lib- 

 ■eral proportion of oligoclase intermingled. In form the individuals 

 are allotriomorphic; they give the typical, broadened gabbroid form 

 as distinct from the idiomorphic or diabasic form of feldspar in- 

 dividuals seen in the diabase boulders just described. These feld- 

 spars are plentifully strewn with many gas and liquid inclusions 

 as well as crystalloids of different minerals. 



Augite, diallage and hypersthene occur in these gabbros; the 

 two former, in the slides examined, are present in about equal 

 proportions. There is no special feature to mention touching 

 these minerals, save their proneness to alter into hornblende. 

 Everywhere a rim of this mineral can be seen around the pyroxe- 

 nes. As a rule the normal crystallized hornblende appears as the 

 resultant product, although at times a fibrous modification is the 

 one present. Augite insensibly gives place to hornblende as the 

 alteration pr(»ceeds, as is shown in Fig. 3, PI. in. The two miner- 

 als are seldom oriented together, as is seen to be the case in the 

 figure just mentioned. In other places areas are partly diallage 

 and partly hypersthene — with a reaction zone between. Along 

 this reaction zone there lies a fibrous hornblecde. Every consid- 

 eration which position and contact relations can give points to the 

 •derivation of the diallage from the hypersthene; yet the mutual 

 relations of these minerals to the normal composition of the gab- 

 bros of the Northwest preclude the assumption that such derivation 

 has here actually taken place. 



Biotite is frequent in these rocks, and the conditions suggest 

 that it is derived from the hornblende. Figure 3, Plate in, suggests 

 such derivation. Pyrite is present in decidedly subordinate quantity. 

 Magnetite is nearly everywhere present. It appears most prom- 

 inently in the normal gabbro as an incidental alterations product from 

 hypersthene, although in many other associations it is prominent. 

 In places it is said that magnetite appears in large proportion. 

 Exposures three miles south of Rhinelander, on the Merrill wagon 

 road, are reported, where considerable quantities of magnetite ore 

 are seen. Some assays of this ore have been made; selected ma- 

 terial gives 68 per cent, metallic iron, while one assay of what was 

 -called an average sample was furnished by Mr. John Doherty of 



