Gabbroid Rocks of Minnesota. — \'\ 'inchell. 357 
In the following table is given for comparison a series of 
analyses of rocks, more or less similar, all derived from gab- 
bro areas : 
I II III IV V VI VII 
SiOj 21.50 4.08 22.87 21.25 24.74 i6.r7 14.95 
TiOo trace 14.25 g.gg 6.30 9.53 7.14 8.50 
AlgOg..,. 8.02 6.40 10.64 5-55 6.99 5.34 8.95 
FcgO,... 30.54 3343 Fe^O^ Fe;,0, 24.58 35.20/ ^„ 
FeO 33.53 34.58 44.88 4345 21.86 23.22 »; -'' 
MnO 32 .45 2.05 .40 .69 .46 .30 
MgO.... 1.95 3.89 5.67 18.30 g.23 7.56 10.25 
CaO 1.55 .65 .65 1.65 2.88 1.84 1.80 
NaaO 42 .29 
1^26 10 .15 
HgO 2.77 1.32 3.05 2.60 .40 1.40 
100.70 99.71 100.00 99.66 100.61 98.42 99.14 
Sp. Gr. 4.00 
-4.04 
I. Silicoferrolyte (96c) from Bi'-ch lake, Minn. No appreciable 
traces of BaO, nor SrO, nor Lil'O. 
II. "Magnetite-SpinelHte," from Roiitivara, by W. Petersson: 
Geol. For. Forh. B. 15. p. 49. 1893. With .20 Cr^Oa and .016 P0O5, 
Ref. W. C. Brogger: Das Ganggefolge des Laurdalits. 1898. p. 321. 
III. Magnetite-olivinyte from Iron Mine Hill, Cumberland, R. I.. 
described by M. E. Wadsworth; analysis quoted by Vogt: Bildung 
von Erzlagerstatten durch Dif^erentiationsprocesse in basischen 
Eruptivmagmata. Zeit. f. prak. Geol. I. 1893. Includes also .20 Cu 
and traces of S and P:;0-,. 
IV. Magnetite-olivinyte from Taberg, Sweden, described by 
Tornebohm: Geol. Foren. Forhand. B. 3, 1876; B. 5, 1881; B. 6, 1882. 
Analysis quoted by Vogt. Ibid. Includes also .127 P^Os, .013 S, .o-' 
Cu, and trace of V2O5. 
V. Titanrferous iron ore in olivine diabase, etc. from Ulfo; analy- 
sis quoted by Vogt Ibid. Includes also .07 Pn O5, .03 Cu, and .01 S. 
VI. Magnetite-olivinyte from Inglamala, Sweden; analysis quoted 
by Vogt. Ibid. Includes also 1.02 S. .40 Vi^Oo, .07 P:;Oo, and trace of 
Cu. 
VII. Magnetite-olivinyte from Langhult, Sweden; analysis quoted 
by Vogt. Ibid. Includes also .it8 P2O5, and .019 S. 
The Minnesota type is entirely exceptional in its very low 
per cent of titanium. The small amoimt of magnesia and of 
calcium will be remarked at once; further, as in nearly all 
similar rocks* there is an excess of alumina which ex- 
*The exceptions are very rare: See Vogt: Zeit. f. prak. Geol. I. 
1873, and Kemp, who has recently studied, tlie composition of these 
rocks in an article entitled: Titaniferous Iron Ores of the Adirondacks, 
19th Ann. Rep. U. S. Geol. Survey, part III, p. 377. 1899, 
