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THE BASIC MASSIVE ROCKS, ETC.
697

Kloos[1] found 1.61 per cent of potash in the rock, which they very properly regarding as belonging to orthoclase. The spaces between the feldspars are filled with a diallage which is always more or less altered to greenish uralite. The alteration in many sections is carried beyond uralite to chlorite. The magnetite is very large, abundant and titaniferous. Apatites of large size are found in all sections. Biotite is not an uncommon accessory. Olivine is absent from all section."

It is very evident that the writer is not describing by these words the rock of the great 'flow' as he defined it in his later papers, but that he is dealing exclusively with the orthoclase gabbros, which were afterwards separated from the underlying mass and given a position just above this.[2]

The only specimen of the true basal gabbro examined by Irving[3] came from the Cloquet river, in Sec. 34, T. 53 N., R. 14 W. in Minnesota. This he characterizes as "A very fresh olivine-gabbro. It is light gray in color, very coarse grained, and [is] composed chiefly of very fresh plagioclase (anorthite). Quite fresh diallage fills in the space between the feldspars. A few large fresh olivines occur here and there in the section. Titaniferous magnetite is abundant, and large sized, and biotite occurs in a few small scales."

Dr. Wadsworth[4] made no attempt to describe the general features of this great mass of rock. His descriptions are of hand specimens furnished him for examination by the officers of the Minnesota survey. Among them were several representatives of the "basal flow,"[5] but these were not studied with reference to each other, except in regard to their alterations.

  1. ↑ Neues Jahrb. f. Min., etc., 1877, p. 113.
  2. ↑ See ante, p. 692.
  3. ↑ Copper-Bearing Rocks, p. 272, also p. 46.
  4. ↑ Geol. and Nat. Hist. Survey of Minnesota.Bull. No. 2.
  5. ↑ The specimens described by Dr. Wadsworth that are thought to belong to the basal gabbro are the following: No. 696, p. 69; 706 and 702, p. 70; 773 and 713, p. 71; 699, 769 and 701, p. 72; 689 and 721, p. 75; 780, p. 85; 707, p. 87; 693, p. 88; 694, 704 and 703, p. 89; 787, p. 90; 715, 692 and 777, p. 91; 691, p. 92; 700, 714 and 698, p. 93; 705, p. 94; 514 and 513, p. 95; 697 and 776, p. 96; and 781, p. 97.
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