PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 391 



Gabbro.] 



The gray portion consists essentially of augites and plagioclase. The former are 

 small, much altered and earlier than the latter. A little magnetite and a few grains 

 of biotite, with a few isolated small areas of quartz, due to the proximity of the acid 

 rock and much apatite, make up the category of minerals of this portion of the slide. 

 The quartz does not appear in this portion in the form of micropegmatyte, but as 

 isolated sizable grains. -Rods and spicules, as well as irregular accumulations of 

 magnetite seem to constitute the chief impurity in the augites. Still they are 

 frequently simply clouded with alteration products in such a manner that their 

 nature cannot be determined. One section. 



Age. Cabotian. N. H. w. 



No. 512. GABBRO. 



N. E. J^ sec. 25, T. 50-15, on the "Herman Town road," northwest from Duluth. Land of Peter Benson. 

 Occasional exposures of this rock rise above the rolling surface of gravelly red till. Elevation here is 500 feet, 

 more or less, above lake Superior. 



Kef. Annual Report, x, page 35; Bulletin ii, page 91. 



Meg. Heavy, gray, trappean rock with so much magnetite that it sometimes 

 disturbs the needle, rather coarse grained. 



Mic. The feldspar is not wrapped about by the pyroxene, but was generated 

 about cotemporary with it, presenting the structure of gabbro. It has an extinction 

 angle on 010 of 21 which is about that of labradorite; another has extinction at 

 26, at Idbrador-bytownite. Extinction on a section nearly perpendicular to n s is 15J 

 to 17|, which is rather low for labradorite, but not conclusive, since the obliquity of 

 the section may cause this deviation. 



The pyroxene has a fine, clear, lamellar cleavage, like the cleavage seen in No. 

 133, but in the whole slide but two or three augite grains can be found whose 

 extinction, like that of the augite of No. 133, is parallel to this cleavage. It occa- 

 sionally shows twinning bands similar to those of albite in the plagioclase. Pyroxene 

 forms rims about the olivine. 



Olivine in subordinate amount occasionally surrounds the magnetite grains. 

 In date it preceded the pyroxene. In some cases it has been replaced by ferruginous 

 products.of decay (bowlingite). A hematitic stain is a common feature in the vicinity 

 of the olivines. The section studied is so thick (about .055 millimeters) that the 

 olivine shows the light tints of the fourth order, whereas the augite is highly colored 

 and about its edges all of the lower orders can be counted. In another section, made 

 thin, the same fact in respect to the relative ages of the olivine and the feldspar is 

 seen as in No. 258, viz.: it is in some cases later than the feldspars and than the 

 augite. (Compare No. 703.) 



Apatite is visible in sizable grains of the first consolidation. They are rounded 

 at the extremities, and elongated, and present a marked shagreen on lowering the 



