1 88 The American Geologist. September, 1900 
Plate XVIII 
Figures 10 and 11. Apatite crystals deeply corroded, in the 
orthoclase gabbro (1797) from Duluth; the apatite contains many 
inclusions. It is surrounded by orthoclase, hornblende, and mag- 
netite. X 120. 
Figure 12. Corroded and fractured apatite in orthoclase gabbro 
(1797) from Duluth. The apatite (6) contains masses of orthoclase 
(i), magnetite {2^) and biotite (10), besides many liquid inclusions. 
It is surrounded by orthoclase and magnetite, x 120. 
Figure 13. Crystals of potassium platinic chloride formed by ad- 
ding chloride of platinum to a solution of the abnormal apatite from 
the orthoclase gabbro (1797), from Duluth. x 530. 
Figure 14. Crystals of sulphate of potassium and bismuth formed 
by adding bisulphate of bismuth to a solution of the abnormal apatite 
from the orthoclase gabbro (1797) from Duluth. x 500. 
Figure 15. Cavities of unusual form in apatite of the orthoclase 
gabbro (1797) from Duluth. A. Three ellipses with a common 
center. B. A double border. C. Two cavities united by a curved 
canal. 
Figure 16. Halo in biotite along the contact between biotite and 
enstatite, in the cordierite noryte (983) from Sec. 15, T. 63-9. x 100. 
Figure 17. Twinning of cordierite, first type; the twinning axis 
is the vertical axis, and the composition face is M (no). In the 
figure the part in full lines was drawn with a camera lucida from a 
section of the cordierite noryte (983) from Sec. 15. T. 63-9. x 300, 
Figure 18. Twinning of cordierite, second type; the twinning 
axis is the vertical axis, and the composition face is g" (130). The 
part in full lines was drawn with a camera lucida from a thin section 
of the cordierite noryte (983) from Sec. 15, T. 63-9. The figure also 
shows the change to twinning by interpenetration. x 150. 
Figure 19. Orientation of staurolite in cordierite, in the cordi- 
erite noryte (983) from Sec. 15, T. 63-9. The figure shows the face 
h^ (100) of cordierite and the face g^ (010) of staurolite. x 250. 
Figure 20. Intense halo in penninite about a crystal of zircon. 
In the quartz gabbro (854G) from near Snowbank lake. The re- 
fringence and birefringence are both increased in the halo, x 300. 
Figure 21. Magnetite preserving the trace of the cleavages of 
a pyroxene, now altered to penninite, in the quartz gabbro (854G) 
from near Snowbank lake, x 60. 
Figure 22. Halo in penninite about a mass of rutile needles, in 
the quartz gabbro (854G) from near Snowbank lake, x 300. 
Figure 23. Amorphous bowlingite, still showing the incomplete 
and irregular form of the original olivine. In the quartz gabbro (954G) 
from near Snowbank lake, x 60. 
\To be continued. ^ 
