PERIDOTITE INTKUSIVES. 253 



The purest form of peridotite is wehrlite, which is composed essentially 

 of olivine and augite. When, besides these minerals, hornblende is present 

 in large quantities, the rocks belong to the amphibole-peridotite type. In 

 some specimens biotite is almost in sufficient abundance to warrant the 

 naming of them biotite-peridotite. Again, in other specimens feldspar is 

 present in considerable quantity and the rock approaches an olivine-gabbro 

 or olivine-hornblende-gabbro. 



WEHRLITE. 



This is represented by a coarse-grained rock which is mottled and has 

 a dark-green color. (Specimen 23763, from sec. 22, T. 42 N., R. 31 W., 

 N. 1,500, W. 900 paces.) Under the microscope the mottling is seen to 

 be due to the association of very dark greenish-black serpentine pseudo- 

 morphs after olivine with a light-colored augite. 



Olivine and augite were present in about equal quantities. They are 

 in anhedra, and therefore must have crystallized at about the same time. 

 The olivine is, with very few exceptions, completely altered to serpentine. 

 Augite has a very poor development. Between the olivine and augite are 

 small quantities of irregular plates of biotite. A few small irregular 

 pieces of a very light colored greenish hornblende were observed. They 

 are intergrown with the pyroxene and give it an imperfect poikilitic texture. 



This wehrlite is unquestionably the same as specimen 1247 of the 

 Geological Survey of Wisconsin, described by Dr. A. Wichmann as serpen- 

 tine, consisting chiefly^ of serpentine with some unaltered olivine and 

 augite. 



AMPHIBOLE-PEEIDOTITE. 



This variety of peridotite was obtained from the outcrop N. 1,260, W. 

 200 paces from the southeast corner of sec. 29, T. 42 N., R. 31 W., on the 

 east bank of the Michigamme River. The rock is very coarse grained, 

 and possesses poikilitic texture. It is composed of hornblende, pyroxene, 

 olivine, biotite, and iron oxide. The hornblende equals in quantity all of 

 the other constituents. Some of the hornblende individuals measure 3 cm. 

 in length, and include all of the other constituents except the biotite. The 

 pyroxene and olivine seem to have crystallized at about the same time, as 



' Microscopical observations of the iron bearing (Huronian ) rocks from the region south of Lake 

 Superior, by Dr. Arthur Wichmann, Leipzig, 1H76: Geol. of Wisconsin, Vol. Ill, 1880, p. 619. 



