PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 387 



Diabase.] 



have been eruptive since the Keewatin, i. c. unless those granites be themselves 

 derived from fusion of Keewatin rocks, and such a material escaped fusion. (Compare 

 H. V. Winchell, Geological Age of the Saganaga Syenyte, American Journal of 

 Science [3], xli, 386.) N. H. w. 



No. 501. DIABASE. 



Right bank of the St. Louis river; centre of sec. 19, T. 49-16 W. 

 Ref. Annual Report, x, pages 26, 28, 29. 



Meg. A black, heavy, rather fine-grained diabase. 



Mir. The section shows feldspar laths, <ni<jitc, magnetite and alteration products. 

 The feldspar is abundantly twinned according to the albite law, and a few grains are 

 seen which show this twinning and also that according to the pericline law. Equal 

 extinction angles on either side of the composition faces of the albite twins run up 

 as high as 29, and a grain which grave a negative bisectrix yery nearly perpendicular 

 had an extinction of 63; both of these results indicate h<l>r<tdi-ife. The labradorite 

 is altering in places to a cloudy brownish material. The augite, instead of being in 

 small plates, as would seem to be the case when viewed in ordinary light, in polarized 

 light is seen to be in grains, which, however, are usually later than the labradorite. 

 Some of the augite appears to be earlier than, or about the same date as, the feldspar. 

 The augite is altering to a greenish material, mostly chlorite. Much magnetite is 

 present in small grains and crystals, probably original, and in rough rods, probably 

 secondary. 



One section. 



Age. Cabotian; dike in Keewatin(?) rocks. u. s. G. 



No. 502. DIABASE (with olivine). 



Near Knife falls, at a cut by the railroad; width and direction not easily seen. 

 Ref. Annual Report, x, page 28; Bulletin ii, page 112. 



Meg. Rather coarse-grained and dark-gray rock. 



Mic. The feldspar is embraced ophitically by the larger augites, but there are 

 many augites, of smaller size, that preceded the feldspar. A little hornblende has 

 been formed by alteration of the augite, also a little epidote- from the same source. 

 The olivine is considerably altered also, becoming a brownish or yellowish brown 

 boicltitgite. Magnetite is common as rods and shapeless aggregations. 



Two sections. 



Aye. Cabotian; dike in Keewatin (?) N. H. w. 



No. 503. DIABASE (with olivine). 



Prom another dike near the same place as No. 502. 



Ref. Annual Report, x, page 28; Bulletin ii, pages 105, 106. 



Meg. Rather coarse-grained and dark -gray rock. 



