PART II. 



PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 



No. 1. GABBKO. (Rice Point Granite.) 



N. W. 14 sec. 34, T. 50-14, Duluth. 



Ref. Annual Report ix, pages 11, 18, 32; Annual Report x, pages 41, 72, 139, 201, 204; Bulletin ii, pages 73-75; 

 American Association for the Advancement of Science, vol. xxx, page 162, 1882; Final Report, vol. i, pages 196-199; 

 vol. iv, pages 571-580. 



Compare J. H. KLOOS, Zeit. d. deutsch. geol. Gesell, xxiii, pages 440, 441, 1871 (Translation, Annual 

 Report x, pages 193, 194) ; A. STKKNG and J. H. KLOOS, Neues Jahr. f. Min., etc., 1877, pages 113-117 (Transla- 

 tion, Annual Report xi, pages 51-54); N. H. WINCHELL, Annual Report viii, pages 22-26, 1880, and Final Report, 

 vol. i, pages 147, 149, 1883; R. D. IRVING, Mon. v, U. S. Geol. Survey, page 55, 1883; C. L. HEBRICK, E. S. CLARKE 

 and J. L. DEMING, American Geologist, vol. i, pages 342-344, 1888; W. S. BAYLEY, Journal of Geology, vol. i, 

 pages 696, 697, 1893; vol. ii, No. 8 and vol. iii, No. 1. 



Megascopic characters. This rock varies considerably in color and grain, but as 

 a rule is coarse grained, lavender gray or greenish gray and composed largely of 

 feldspar whose twinning striations are very evident and which is usually crystallized 

 in a uniform massive structure, but is sometimes distinctly porphyritic. Its tabular 

 crystals are usually less than half an inch across, but one of our specimens exhibits 

 a single crystal which has a roughly rounded outline and a diameter of an inch and 

 a half on its cleavage face (compare, also, No. 128). Sometimes a lighter-colored 

 feldspar with rectangular cleavages is sparsely scattered amongst the gray crystals, 

 and this sometimes is whitish, and in other places pinkish, and in proximity to areas 

 of red rock (No. IB) these red feldspars are abundant. With such aspect this rock 

 has received the name "orthoclase gabbro." Between the plagioclase grains, but 

 making only a small part of the rock, are pyroxene and magnetite. The pyroxene 

 has an irregular, though pronounced, cleavage-parting. It is usually in xenomorphic 

 anhedrons, but occasionally it embraces the feldspar crystals ophitically. The 

 magnetite is brightly metallic on fresh surfaces; on weathered surfaces it stands out 

 rigidly after the other minerals have disintegrated. It is sometimes in tabular and 

 other abnormal forms, owing apparently to the shapes of the cavities which it 

 filled after the solidification of the other minerals. 



In proximity to the lighter-colored feldspars are occasional nests of epidote and 

 a rare crystal of pyrite. When the two feldspars are in contact the red coloration 

 sometimes slightly penetrates the plagioclase crystal. 



While this rock, as a whole, is coarse grained and uniform in texture, the large 

 tabular plagioclases indicate two periods of crystallization. Still, the sizes vary 



