242 



THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



I'roiu llie sections of these rocks, but when present it is very rare, and occurs 

 in small irregular grains not uncommonly intergrown with the hornblende 

 and evidently older than the hornblende. It is light green in color, with a 

 scarcely noticeable pleochroism. Its raonoclinic character was readily deter- 

 minable; but a more exact determination was not made. It does not, how- 

 ever, show diallagic parting, and is diagnosed as possibly diopside. 



The feldspar shows the best development of the accessory minerals. 

 It can rarely, however, be said to be automorphic. The texture is, on the 

 whole, granular. 



From a mineralogical study of the rocks alone, one would unhesitat- 

 ingly place them with the diorites, especially if those facies were seen in 

 which the pyroxenic constituent was wanting. 



The following analysis (Sp. 23354), obtained from Mr. George Steiger, 

 of the United States Geological Survey, shows the chemical composition 

 of one of these rocks : 



Analysis of hornhlende-gabbro. 



SiOi . 

 TiO, . 



FejOs 

 FeO . 

 MnO. 

 CaO . 

 MgO. 



49.80 

 .79 



19.96 



6.32 



.49 



11.33 

 7.05 



K,0 



Na-iO 



H2O lOO-''— . 

 H.2O 100'=+. 



P2O5 



CO2 



Total 



.61 

 2. 22 



.13 

 1.71 



.07 



.15 



An examination of the analysis shows that the microscopical determi- 

 nation of the rock as a diorite would be incorrect if we accept, as has been 

 done in the preceding pages, Brogger's characterization of the diorite and 

 gabbro families.^ 



The rock analyzed is hornblende-gabbro, as shown by the relatively 

 high content hi the characteristic alkaline earths, especially magnesia, 

 which usually appears in inverse proportion to the silica, and in the low 

 percentage of alkalies. 



' Op. cit., Part II, pp. 35, 39. 



