Geological Excursion into Central Wisconsin — Hall. 25S 



planes have allowed the infiltration of hornblendic material; 

 kaolin as a result of their decomposition is scarcely seen. The 

 augite has suffered severely; indeed only a few cores of this min- 

 eral remain. Hornblende has very generally taken its place. This 

 last named mineral is of the green variety, and is sometimes fi- 

 brous and sometimes more compact, with well developed cleavage. 

 The manner in which the fibres of hornblende have penetrated the 

 labradorite crystals, especially in the direction of the axis c, is in- 

 teresting. They enter the feldspars only at their ends and at cer- 

 tain cracks where the largest and longest feldspars have been broken. 



Magnetite occurs in small crystals and crystal clusters in 

 sufficient quantity to be separated from the powdered mineral 

 by use of the magnet. The chief interest attached to this mineral 

 lies in the segregating around it of folia of biotite, which mineral 

 away from the immediate vicinity of magnetite scarcely appears. 

 With the quantity of ferric oxide in ordinary augite at 8.75 

 per cent.,* in green hornblende near 6 per cent.,t and that in bio- 

 tite at about 11^ per cent., J and with the basic constituents of the 

 rock profoundly altered, it would seem clear that an oxide of iron 

 must be separated out with the secondary development of biotite. 

 Such development of this mineral as well as of hornblende 

 seems clearly proved. But few other minerals were noted, and 

 those apparently accessory — the chief of these were apatite in 

 long minute needles, and epidote in equally minute granules, scat- 

 tered through both labradorite and hornblende, but more partic- 

 ularly the latter. 



It may be mentioned here that at Grandmother Palls the 

 darker colored rock is regarded as completely altered diabase. 

 This rock is described further on in this paper. 



Gahhro. In geographical distribution the gabbros and the 

 gabbroid rocks occupy four different and disconnected localities, 

 viz: around Rhinelander, Merrill, Wausau and Mosinee. As 

 there seem to be certain points of difference in the typical sam- 

 ples of these localities they will be briefly described separately. All 

 save those at Mosinee are hypersthenic gabbros. 



At Rhinelander, between the Milwaukee, Lake Shore & West- 

 ern railway tracks and the river south of the railway bridges is a 



*Note Dana's tables, System of Mineralogy, 5th edition, p. 218. 

 tibid, p. 237. 

 • JIbid, p. 305. 



