Geological Excursion into Central Wisconsin— Hall. 259 



quently, transformations in mineral constituents and in structure,* 

 which the original group of substances has undergone. Through 

 the disguise under which they now present themselves there can be 

 seen all the essential characters of rocks precisely similar to those 

 studied at Rhinelander and Wausau. The reasons why these rocks 

 are more changed than those were not traced. They may be sur- 

 mised to lie in the nearness to, or even contact with rocks of quite 

 different constitution. Such contact metamorphisra has been 

 noted in hundreds of places throughout the world. 



There is one phase of these Merrill gabbroids which cannot 

 be passed by unnoticed. Scarcely seen with the unaided eye, under 

 the microscope it is the most conspicuous feature in their mineral 

 structure. It is the* semblance of the eye structure which so 

 abounds in the acidic laminated rocks, and it seems to spring from 

 a segregation into nests of the hornblende and the feldspars, all of 

 which minerals are secondary. The basic constituents, and horn- 

 blende is the chief, are arranged around the nests of feldspar 

 granules. But among the latter the peculiar requisite of the eye 

 structure, viz: a core of rounded lenticular feldspathic material,f is 

 lacking — all individuals are minute in size, fresh looking and limpid. 

 Every character of these rocks seems to show that one of the prime 

 causes of their present condition was pressure. 



The Gneisses, Crossing the bridge to the west side of the 

 river from the knobs of gabbro, in Rhinelander, one finds at the 

 intersection of the railway with a wagon road, a typical mass of 

 hornblende biotite gneiss. The color is dark; the texture medium; 

 the lamination strong and somewhat contorted; the general direc- 

 tion is N. 25*^ E., with a southeasterly dip of 60^. Lenses of quartz 

 are not infrequent, and true granitic veins occur. In addition to 

 these, one sees masses consisting largely of hornblende of medium 

 texture and with some parallelism of structure. 



Microscopically, the gneiss presents no peculiar characters. 

 The quantity of hornblende is very small in proportion to the bio- 

 tite, and the proportion of plagioclastic feldspars is very large. In- 

 deed, orthoclase is scarcely seen. The hornblendic masses just men- 

 tioned are very fresh in the condition of their constituents, horn- 

 blende being the chief and plagioclase abundant. The twinning 

 striae of the plagioclases are sharp and strong. A pyroxene is un- 



*Pre8twich, Geology, Chemical, Physical and Stratigraphical, Vol. i, p. 397. 

 tCompare Zirkel F., Lehrbuch d. Petrographie, Vol. ii., page 414. 



