THE BASIC MASSIVE ROCKS, ETC. 697 



Kloos 1 found 1.61 per cent of potash in the rock, which they 

 very properly regarded 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 sections." 



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. Titanif- 

 erous 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^.92; 700, 714 and 

 698, P- 93 ; 705, p. 94 ; SM and 513, p. 95 ; 697 and 776, p. 96 ; and 781, p. 97. 



