186 THE GEOLOGY OF MINNESOTA. 



[Diabase. 



proportion, and in so large crystals, that the augite simply fills angular spaces between 

 the feldspars. The rock also contains magnetite, a little chlorite, biotite and olivine. 

 Af/e. Cabotian; Beaver Bay diabase. N. H. w. 



No. 122. DIABASE (?) (GiviuiliUc. c/dbbro?) 



PLATE I, FIGURE 4. 



On the inside and east side of the second small bay above Beaver bay entrance. Columnar. 

 Ref. Annual Report, ix, page 32. 



Meg. A fine-grained, granular, gray rock, compact and fresh. The minerals 

 composing the rock cannot be made out macroscopically. The rock has a "sugary" 

 appearance, reminding one of the rocks to which the term "muscovado" has been 

 applied (see Twenty-first Annual Report, pages 143-152). 



Mir. The rock is composed essentially of feldspar, pyroxene and magnetite. 



The feldspar is by far the most abundant mineral. It occurs in grains and 

 crystals varying from those which are nearly idiomorphic and lath-shaped to 

 completely allotriomorphic individuals. Under a low power these lath-shaped 

 feldspars appear quite prominent and are sharply defined, but under a high power 

 their outlines are seen to be usually only partly idiomorphic and frequently allotrio- 

 morphic. The feldspars which do not approach the lath-shaped form are usually the 

 larger in size, and not uncommonly these allotriomorphic individuals are of consid- 

 erable size and embrace the pyroxene and magnetite, as well as the lath-shaped 

 feldspars in part, in a-poikilitic manner (plate I, figure 4). The feldspar is commonly 

 abundantly twinned according to the albite law, but the larger grains are not usually 

 as abundantly twinned as are the lath-shaped crystals, but no fixed distinction can 

 be drawn between them. Equal extinction angles one either side of the composition 

 face not uncommonly run up as high as 27 or 28, but were not noticed higher than 

 this. A section furnishing a positive bisectrix perpendicular, gave an extinction of 18 

 while another showing the negative bisectrix gave an angle of 62. All of these 

 results point to labradorite as the feldspar of the rock. 



The pyroxene, which was earlier than the microlitic feldspars, is in small, more or 

 less roundish grains, and is not idiomorphic. Cleavage is not well developed, but a 

 fine parting is distinct, and the mineral can be referred to dial I a (jr. It often contains 

 numerous magnetite inclusions and is altering to a greenish yellow fibrous mineral. 



Magnetite is quite common in grains of about the same size as the diallages. 

 Apatite is also common, both in minute short prisms and in slender needle-like forms. 

 A little biotite, evidently secondary, is also present. 



As to order of consolidation of the minerals: It is clear that the magnetite and 

 diallage preceded the labradorite. This mineral, both the lath-shaped and the allo- 

 triomorphic individuals, includes the first two minerals. The lath-shaped labrador- 



