PETKOGRAPHIC GEOLOGY AND DESCRIPTIONS. 473 



Gabbro. Apotrachyte.] 



feldspar, which is probably near labradorite in composition. In one part of the 

 section is a small area composed of smaller lath-shaped plagioclases and small, more 

 or less rounded augite and magnetite grains. 



One section examined. 



Af/e. Cabotian. u. s. G. 



No. 652. GABBRO (ivith hm-nblende). 



"Prom a point about one-fourth mile from the Brule' river and half a mile west of the town line." Evi- 

 dently from near the centre of N. }- sec. 36, T. 63-1 E. 



Ref. Annual Report, x, pages 68-70; Bulletin ii, pages 102, 103. 



Meg. A medium-grained, dark-gray, granitoid rock composed of gray feldspar 

 and much black material. 



Mic. The following is M. E. Wadsworth's description of this rock:* 



"A grayish-brown crystalline aggregation of feldspar, augite and magnetite. In 

 the section the feldspar is partly altered and replaced by kaolinized material inter- 

 stitially arranged with graphic quartz. Part of the feldspar is distinctly plagioclase, 

 and part of the augite has been changed to brown hornblende and brown biotite. 

 Apatite rods and microliths are to be seen associated with the graphic or eozoon 

 quartz. " 



The main mass of the feldspar is plagioclase, and equal extinction angles in 

 sections normal to 010 run up to 33, indicating a feldspar as basic as labradorite. 

 Another section was made. This shows the presence of some olivine which has con- 

 siderably altered to a yellowish-brown substance perhaps boiclingite. 



Two sections. 



Age. Cabotian. u. s. G. 



No. 653. APOTRACHYTE. 



"From the Brule", some west of the town line." Evidently from the S. E. y sec. 25, T. 63-1 E. 

 Ref. Annual Report, x, page 68. 



Meg. A dark, brownish-red rock with small, porphyritic red feldspars, and a 

 little of a yellowish material. The groundmass of the rock is very fine grained. 



Mic. The section shows much reddened, almost opaque, porphyritic feldspars 

 in a groundmass which is also reddened, consisting of quartz, feldspar, hematite, mag- 

 netite and a greenish alteration product. Under crossed nicols the groundmass breaks 

 up into the irregular patches of poikilitic quartz which are so frequently found in 

 the "red rocks" of northeastern Minnesota. 



One section examined. 



. [<je. Cabotian. 



Renxirk. This rock is very similar to No. 647. See, also, figure 3 of plate I for 

 the " patchy " groundmass. u. s. G. 



* Bulletin ii, pp. 102, 103. 



