654 SIR A. GEIKIE AND ME. J. J. H. TEALL ON THE [Nov. 1 894, 



especially in Scandinavia and North America. 1 Here, as in most of 

 the other allied cases, the iron ores are titaniferous. The case 

 which comes nearest to the one under consideration is that described 

 by N. H. & H. V. Winchell in Bulletin VI. of the Geological Survey 

 of Minnesota, p. 126. Speaking of the gabbro of the Mesabi 

 Range, the authors say : — " Occasionally a gneissic (laminated) 

 structure is seen in it. Such occurs on the east shore of Birch 

 Lake where a conspicuous hill is marked by parallel weather-lines 

 sloping towards the south, the lines being due to the weathering-out 

 of the contained olivine, which is disseminated in alternating sheets 

 of greater and less prevalence throughout tbe hill. . . . This gneissic 

 structure is not due to shearing-pressure nor to sedimentation, but 

 to a varying abundance of the more easily decaying minerals, such 

 variation occurring in sheets, and on the weathered edges appearing 

 as depressed lines or grooves." The writings of other authors con- 

 tain references to gneissic structure in the gabbros and norites which 

 are associated with ore-deposits ; and Vogt figures such structures 

 in a dyke of ilmeuite-norite at Storgangen in Norway (Geol. Foren. 

 Forh. vol. xiii. 1891, p. 496). 



The Coarse-grained Massive Gabbros. — The constituents of these 

 rocks are the same as those of the banded gabbros, except that olivine 

 has not been observed. The uralitization of the augite has often taken 

 place to a greater extent than in the members of the last group, and 

 with the change in the character of the ferro-magnesian constituents 

 is associated a change in the colour of the rocks. The normal 

 gabbros are brown or black ; the uralitized gabbros green. The 

 felspars in some of the uralitized gabbros show marked signs of 

 having been fractured and broken ; but this action has been local in 

 its character, and may possibly be connected with the molecular 

 changes in the augite. The rocks of this group are much more 

 uniform in composition than those of the banded series. Neverthe- 

 less variations in the relative proportions of the different consti- 

 tuents do occur. The specific gravities of three specimens are 

 respectively 2-82, 2-97, and 3-06. 



The later Gabbro-veins. — These rocks are composed of the same 

 constituents as those of the last group ; but the felspar is, as a rule, 

 far more abundant, so that they are lighter in colour. Inclusions of 

 the fine-grained granulitic gabbro occur in the veins. Apatite, 

 which has not been noticed in the rocks of the other groups, is 

 easily recognizable in the veins. In the few specimens from which 

 microscopic sections have been prepared hornblende predominates 



1 See A. Sjogren (Geol. Foren. Forh. rol. iii. p. 42, 1876, and vol. vi. p. 264, 

 1882); Tornebohm (ibid. vol. v. p. 610, 1881) ; Vogt (ibid. vol. xiii. 1891); 

 Wadsworth (Lithological Studies, Mem. Mus. Comp. Zool. Harvard, vol. xi. 

 ] S84) ; Winchell (Minnesota Geol. Surv. Bull. vi. 1891) ; Adams (' Ueber das 

 jSTorian oder Ober-Laurentian,' Neues Jahrbuch, Beilage-Band viii. 1893). An 

 ore-deposit, consisting of magnetite and pyroxene associated with a peculiar 

 nepheline-pyroxene rock (jacupirangite), has been described by O. A. Derby, 

 Amer. Journ. Sci. ser. 3, vol. xli. (1891) p. 311. 



