Gabbroid Rocks of Minnesota. — Winchell. 303 
is richer in iron, and contains a remarkably large amount of 
magnesia. A feature of the analysis, pointing to peculiar con- 
ditions of origin, is the complete reversal of the ordinary ratio 
between the sodium and potassium. The very high excess of 
alumina, while of course partly explained by the presence of 
biotite, suggests that cordierite was perhaps overlooked in the 
optic examination of the rock. 
I. 
11. 
III. 
IV. 
SiO, 
52.84 
58.30 
51.30 
46.96 
TiO.> 
trace 
.62 
Al.Os 
23.62 
22.55 
25.20 
14 13 
^e.O, 
.65 
2.24 
2.91 
.76 
FeO 
10.00 
7.13 
2.39 
14.95 
MnO 
.43 
.93 
MgO 
3.16 
3.90 
4.01 
15.97 
CaO 
3.92 
1.91 
2.50 
2..32 
Na^O 
2.64 
3.21 
3.82 
.35 
KsO 
.67 
.59 
.79 
1.68 
H3O 
1.87 
.51 
.55 
1-33 
99.80 
100.34 
99.47 
100.09 
Sp. Gr. 
2.81 
to 2.85 
2.84 
to 2.86 
2.88 
I. Cordierite noryte from Sec. 15. T. 63-9, not far from Snowbank 
lake, Minn. No BaO nor SrO; ZrOg and P- O^ not determined. 
II. Average of two analyses of a cordierite noryte from the rail- 
road cut between Gorges and Pallet (Loire Inferieure, France), by 
Lacroix and Pizani. A. Lacroix: Le Gabbro dii Pallet: Bull. 
Carte Geol. France. No. 67, 1899, P- 23. 
III. Cordierite noryte from the quarry of Prinaux, by A. La- 
croix: Ibid, p 23. 
IV. Granulitic hypersthene gabbro. From S. E. 54 Sec. 20, 65-4. 
Described by Bayley. Jour. Geol. III. p. i. Contains hypersthene, 
biotite, diallage, magnetite, and plagioclase. Poor in feldspar. Analy- 
sis by H. N. Stokes; Bull, 148. U. S. Geol. Survey, p. iii. Contains 
also .06 NiOs, .03 P^ O5 and trace of Cr^ Oa. 
It has been noted that the change from gabbro to "mus- 
covadyte" is by absolutely imperceptible degrees; but the. two 
extremes are radically different. In place of the coarse granu- 
lar texture of the gabbro appears a very fine granular-poikilitic 
texture. The coarsely cleavable gabbro is transformed into 
a rock so dense and fi.ne as to be brokenwith great difficulty. 
In structure the massiveness of the gabbro is replaced by more 
or less evident schistosity, this character becoming more pro- 
nounced as the distance from the normal gabbro increases. 
.Vs one passes from the gabbro to the noryte the rock becomes 
