264 Geological Excursion into Central Wisconsin — Hall, 



father falls, or at Morin's farm just given. There is one point of 

 difference, yet this difference must be apparent rather than real; 

 it pertains to the "eye structure" of the rocks. In all three cases 

 there is the peculiar "streaming" or parallel arrangement which 

 other authors* note in the position of the biotite aud hornblende 

 individuals; there is the same fine mosaic of quartz grains, inter- 

 mingled with these basic constituents, and there are the same 

 texture and color. While there the mosaic extends to the very 

 center of the eyes (augeii), here in almost every case the central 

 part is a corroded feldspar crystal. These crystals can usually be 

 identified as plagioclastic. Particles of kaolin and epidote becloud 

 them to some extent and their borders are so changed that they 

 can no longer be traced; in other words, these feldspar cores lie in 

 a segregation of fine grains of quartz, with which are inter- 

 mingled a few impurities such as epidote, hornblende and biotite 

 particles. The attempt has been made to represent this peculiar 

 eye structure in figure 5, plate in, where a sketch of the author's 

 slide No. 884 is given. Perhaps no better description of this 

 slide can be given than Chelius gives of a deformation of granite- 

 porphyry near Eberstadt: "Mutual shattering of the feldspar 

 crystals can often be seen; yellowish-green biotite and green horn- 

 blende individuals wind through the ground-mass in bands around 

 the feldspars, so that a streaming can well be distinguished since 

 the rock microscopically exhibits no flaser-structure. Within nar- 

 row zones between the interjected particles, the grains of the 

 ground-mass show a distinct consecutive arrangement in the di- 

 rection of their longer axis."t 



Crossing the Wisconsin river to the west side from Knowlton 

 a mass of very compact, dark colored, granitoid gneiss is reached. 

 It has a medium texture and is chiefly biotitic in its basic constit- 

 uents, although to the naked eye a greenish color is quite ap- 

 parent. The feldspars are somewhat altered to kaolin and musco- 

 vite; in two or three places calcite was seen to have been deposited as 

 an alterations product — the only instances of the kind noted among 



^Lehmann, Untersuchungen ueb.er die Entsteliung der Altkrystallin- 

 ischen Scliiefergesteine, Bonn, 1884, p. 189. Chelius, quoted by Rbsenbusch, 

 Mikroskopische Physiographie, 2d ed., Stuttgart, 1887, p. 294. Teall, British 

 Petrography, London, 1888, p. 243. The rock Teall described however is an 

 "augenschist." 



■fRosenbusch, Mikroskopische Physiographie der Massigen Gesteine, 

 p. 294. 



