470 THE CRYSTAL FALLS lEOIST-BEARING DISTRICT. 



rounded, bare knobs with almost perpendicular sides, usually situated in 

 the midst of swamps. The main mass of the knobs is a rather fine-grained, 

 slightly schistose, gray rock exhibiting the diabasic structure on weathered 

 siu'faces. On the south sides of the knobs the rock is much denser, and in 

 most cases is much more highly schistose than the main rock mass. 



Under the microscope these rocks present the usual features of schistose 

 dike greenstones. They consist almost exclusively of hornblende, ^jlagio- 

 clase, and quartz. The hornblende, which is the common yellowish-green 

 variety, occurs in long plates and in columnar crystals, some of which are 

 idiomorphic in cross section, and also in slender needles penetrating the 

 quartz and feldspar. These two minerals form an aggregate between the 

 larger hornblendes. The feldspar is mainly a calcium-soda plagioclase, 

 though a small quantity of albite may also be present. It occurs as irreg- 

 ular grains embedded in a mosaic composed of rounded grains of the same 

 feldspar and of quartz, and appears to be a new crystallization subsequent 

 to that of the greater portion of the plagioclase. At any rate, a single 

 large grain often fills the interstices between numbers of the mosaic grains 

 and extinguishes uniforml}^ over large areas. The magnetite in the rock is 

 titaniferous. It occurs in little crystals and in small irregular grains that 

 are often surrounded by a granular zone of leucoxene. 



This rock may serve as a type of nearly all the other dike greenstones 

 in the district under discussion. Some may be more schistose than this one, 

 while a few -mnj be more massive, but in general characteristics they are all 

 similar. The more schistose rocks differ from the less schistose varieties 

 simply in the possession of a greater amount of quartz and a greater quantity 

 of what appears to be newly formed feldspar. Their greater schistosity is 

 due to the more uniform elongation of their components. 



The fine-grained greenstones found on the edges of the coarser-grained 

 ones, and occasionally as independent dikes, are weathered diabases of the 

 normal type. 



COMPARISON OF THE STURGEON RIVER AND THE MARQUETTE CRYSTALLINE 



SERIES. 



The Basement Complex in this area is essentially like that in the Mar- 

 quette district, except that the altered tuffs so abundant in the northern area 

 are absent from that now under discussion. The biotite-schists of the two 



