PETKOGRAPHIC GEOLOGY AND DESCRIPTIONS. 393 



Forellenstein . ] 



Remark. This rock might be styled anorthosyte. By far its largest constituent is 

 feldspar. It is similar to the "feldspar rock" of Carltou's peak, and to that of Little 



Saganaga lake. N. H. w. 



No. 514. FORELLENSTEIN. (Gabbroid.) 



Near the centre of the S. W. % sec. 22, T. 50-15 W., near Duluth. 



Ref. Annual Report, x, page 35; Annual Report, xxiii, page 230; Bulletin ii, page 95, plate V, figures 1 and 2. 



Meg. A dark gray rock of medium grain. Gray feldspar grains and crystals 

 are seen enclosed in an almost black crystalline mass. 



Mic. The section is composed of a granular mass oifeltlspar, olivine and diallage; 

 niayiii'tite and alteration products are also present, although the rock is comparatively 

 fresh. The diallage is in small amount, the feldspar and olivine being the essential 

 minerals. These two minerals mutually interfere in their outlines, having crystal- 

 lized at about the same time. However, much of the olivine is earlier than the 

 feldspar, though not idiomorphic, while occasionally a feldspar grain is seen which 

 conditions the outline of the adjoining olivine. The diallage is in part later than 

 the feldspars and in part of about the same date. It is seen enclosing olivine grains 

 and in the interstices between the feldspars. 



The feldspar is abundantly twinned according to the albite law and rarely a 

 grain shows the pericline twinning. Equal extinction angles on either side of the 

 twinning plane run up as high as 31, and a grain which shows a positive bisectrix 

 closely perpendicular gave an extinction of 20, both of which results indicate 

 labradorite. 



The olivine is in roughly rounded grains and more commonly in aggregates in 

 which the grains have polygonal outlines. It is altering to a yellowish and also to a 

 green serpentine. 



The diallage is only in small grains. It is much clouded by minute black 

 (magnetite) inclusions which are usually arranged in lines parallel with the parting. 

 In places the i>i/iu>-ene extends in a narrow band between the olivine and the feldspar, 

 and occasionally surrounds an olivine grain. 



Magnetite is not very common and nearly all which is present is secondary, 

 occuring in the altered olivines. Three sections. 



Age. Cabotian. 



llnintfk. The diallage is in such small quantities that it can be regarded as non- 

 essential and the name forellenstein can be applied to the rock, which is essentially 

 a granular aggregate of labradorite and olivine. It seems, however, that this forel- 

 lenstein is only a facies of the usual gabbro very rich in olivine and very poor in 

 diallage. Such a facies of the gabbro is not uncommon. It occurs in considerable 

 amount about fifty miles northeast of this locality.* u. s. G. 



* A. H. Ei.FTMAN. Annual Report, xxiii, pp. 224-230. 



