xiv ILLUSTRATIONS. 
PLATE X1IV.—Thin sections of granitic porphyry or granitell ......-.....-.-..-.-.-.------ 
Fig. 1.—From Eagle Mountain, T. 63, R.2 W., Minn. Orthoclase crys- 
tals saturated with corrosion quartz. Scale, 35 diameters. 
Fic. 2.—The same section in polarized light, showing how numbers of 
adjacent quartz particles polarize together. Scale, 35 
diameters. 
Fic. 3.—From vein in gabbro of Rice Point Quarry, Duluth, Minn. 
Orthoclase decomposed, reddened by iron oxide, and filled 
with secondary quartz, which is largely in regularly out- 
lined forms like the quartz of “‘graphic granite.” Scale, 
25 diameters. 
Fic. 4.—From area of red rock in the Duluth gabbro. Drawn with the 
polarized light to show the way in which the larger quartz 
areas and smaller adjoining ones polarize together. Scale, 
25 diameters. 
XV.—Thin sections of augite-syenite, and granitell or granitic porphyry-.- ..---- 
Fic. 1.—Uralitic augite-syenite, from area of red rock within the coarse 
gabbro of Duluth, N. W. 4, Sec. 27, T. 50, R. 14 W., Minn. 
Orthoelase and oligoclase saturated with secondary quartz, 
which forms some large areas; greenish uralite; chlorite 
as an alteration of the feldspars; magnetite. Scale, 20 
diameters. 
Fic. 2.—Angitic granitell from the north shore of Lake Superior, N. E. 4, 
° Sec. 32, T. 56, R. 7 W., Minn. Oligoclase and orthoclase 
in large altered crystals; quartz in large areas, possibly 
primary; augite, mostly altered to a greenish substance, 
occurring in clusters between the large feldspars along with 
smaller feldspar particles. 
Fic. 3.—Red granitic porphyry from the old Ironton trail, Ashland 
County, Wis. Sec. 34, T. 46, R. 1 W. Reddened feldspar 
crystals saturated with graphically arranged secondary 
quartz; magnetite; greenish chloritic alteration-product. 
Scale, 27 diameters. 
Fic. 4.—Augite-syenite pebble from conglomerate at south foot of Mount 
Bohemia, north side Lac La Belle, Keweenaw Point, N. E. 4, 
Sec. 32, T. 58, R. 29 W., Mich. Orthoclase (twinned) and 
labradorite saturated with secondary quartz, both graph- 
ically arranged, and in exceedingly fine lines along the 
cleavage directions; augite altered to ferrite. Scale, 27 
diameters. 
XVIi— Thin sections’ of Sand Stones ys ae ce aes ea 
Fig. 1.—The ‘‘ Nonesuch” copper-bearing sandstone, from the Nonesuch 
mine, S. E.4, See. 1, T. 51, R. 43 W., Mich. Diabasic detri- 
tus; porphyry detritus; abundant quartz fragments evi- 
dently -derived from the quartzes of quartz-porphyry ; 
magnetite; native copper indicated by the deep red color; 
one large pebble of a quartziferous porphyry with a char- 
acteristic quartz-saturated matrix. Scale, 16 diameters. 
Fic. 2.—Sandstone from the bed of Montreal River, N. E. 4, Sec. 20, T. 
47, R. 1 E., Wis. Diabasic detritus; porphyry detritus; 
rare quartzes; calcite matrix. Scale, 27 diameters. 
Fic. 3.—Sandstone from near the Copper Falls Mine, Keweenaw Point, 
Mich. Fragments from the matrix and porphyritic feld- 
spars of a quartziferous porphyry and from an augite- 
syenite, with rarer quartz fragments, magnetite particles 
and fragments of amygdaloid matrix; calcite cement. 
Scale, 37 diameters. 
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