Gabbroid Rocks of Min?iesoia. — Wincliell. 187 
gabbro (1136); 4. Diabase (954); 5- Orthoclase gabbro (1797); 6. 
Olivine diabase (1843); 7. Plagioclasyte (336E); 8. Cordierite 
noryte (983); 9. Quartz gabbro (854G). 
Plate XVL 
Graphic representation of the chemical composition of the rocks 
by the method of Iddings. SiO,. of feldspathic elements — s; SiOo ot 
ferromagnesian elements^s'; ALOs of feldspathic elements==a'; ex- 
cess of alumina^a; Fe203+FeO=f; MgO^m; CaO of feldspathic 
elements=c; excess of CaO^c'; Na^O^n; K-O^k. i. Silicofer- 
rolyte (960). 2. Troctolyte (514). 3. Olivine gabbro (1136). 4. Dia- 
base (954). 5. Orthoclase gabbro (i797)- 6. Olivine diabase (1843). 
7. Plagioclasyte (336E). 8. Cordierite noryte (983). 9. Quartz 
gabbro (854G). 
Plate XVII. 
Figure i. Olivine gabbro (1136) from Birch lake. Magnetite 
(23), automorphic toward augite (11), which is apparently automor- 
phic toward olivine (20). x 60. 
Figure 2. Olivine gabbro (1136) from Birch lake: inclusions in 
the pyroxene (11) containing no twinning; the alteration to biotite 
(10) commences in the pyroxene, but continues in the olivine (20); 
further, biotite is formed by mesogenesis, between the labrodrite and 
the pyroxene and between the olivine and the labradorite (3). x 60. 
Figure 3. Apatite surrounded by a halo in biotite of the olivine 
gabbro (1136). Apatite ;/g>«m biotite. x 300. 
Figure 4 "Polysomat'c" texture in pyroxene from Pigeon point. 
No difiference in the color of the different areas, x 120. 
Figure 5. Pleochroic halo in clinochlore about magnetite (?) 
The halo consists of two bands, the inner one being nearly colorless. 
In olivine diabase from Pigeon point, x 300. 
Figure 6. Crystal outlines of allanite found in the gabbro. (1136) 
from Birch lake, x 550. 
Figure 7. Pleochroic halo about allanite in brown hornblende 
in the olivine diabase (1843) from Pigeon point. Consists of two 
rings exactly circular, the outer one being much lighter. Both are 
sharply defined, x 300. 
Figure 8. Crystal outline of an unknown mineral, in the plagio- 
clasyte (336E) from Carlton peak. The mineral possesses weak 
pleochroism, in yellow tints. The crystal is not definitely oriented 
optically, x 300. 
Figure 9. Augite (11) enclosing olivine (20) in the poikilitic 
texture in the troctolyte (514) from near Duluth. Magnetite (^2^) is 
also enclosed by the pyroxene, x 60. 
