250 GEOLOGY OF EASTERN WISCONSIN. 



are doubtless the portions referred to by Dr. Percival as " interlami- 

 iiated hornblende and mica." 



The rock is very uniform in character at all points exposed. It 

 presents an obscure parallel structure giving rise to a somewhat defi- 

 nite system of cleavage, but traces of distinct bedding were not ob- 

 served. The mass is traversed by extensive fissures which are readily 

 arranged into three groups, the predominant one of which bears IS". 

 W., and the smaller ones E. of K and K of E., respectively, thus 

 dividing the horizon into nearly equal arcs; but none seem to be de- 

 pendent on the cleavage structure of the rock. 



FIG. 18. 





EAST AND WSST SECTION THROUGH THE BERLIN POKPHYBY. 

 1. Porphyry. 2. Potsdam Sandstone. 3. Lower Magnesian Limestone. 



On the south slope of the hill, and within a few rods of the expo- 

 sure of porphyry, occurs a sandstone in which are imbedded masses 

 of the porphyry of various sizes. The sandstone also contains several 

 species of Potsdam fossils, demonstrating the presence of the por- 

 phyry as an island or reef during the deposition of the sandstone. 

 These facts entirely negative the view that these hills were either 

 ejected as an igneous mass, or thrust up as such by upheaval. They 

 are simply projecting points of an eroded formation. 



THE QUAKTZ-PORPHYBY OF PlNE BLUFF. 



Seventeen miles south of Berlin there rises out of the flood plain 

 of the Grand river a conspicuous mass of quartz-porphyry known 

 as Pine Bluff. It ascends by steep, and even precipitous acclivities 

 to a height of one hundred feet, and being entirely isolated from 

 surrounding elevations, and largely bare of soil and vegetation, be- 

 comes a striking object. The rock consists of white, gray and flesh 

 colored crystals of orthoclase, and of glassy feldspar, set in a very 

 hard, gray or black quartz-felsite base. The crystals of feldspar vary 

 in size from three-tenths of an inch in length, downwards, but are 

 rendered conspicuous by contrast of color. The rock is susceptible of 

 very high and beautiful polish, but is wrought with difficulty on ac- 

 count of its hardness. The dip is about 20 to the E. of S. Obscure 

 glacial striae, still preserved, testify to its endurance. Their direction 

 is S. 45 "W. The accompanying profile exhibits its relations to the 



