168 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
Next in abundance to these pebbles are others of a pink to red granite- 
like rock which macroscopically presents a fine-grained, apparently ecrys- 
talline matrix, through which are thickly scattered porphyritic crystals of 
pink feldspar. In the thin sections good sized orthoclases and oligoclases 
greatly predominate. Augite occurs in crystals of some size, which are 
penetrated by bands and needles of opaque brown ferrite, arranged at defi- 
nite angles. There is also a considerable portion of the section in which 
there appears a radial or spherulitic structure, which is either produced 
or emphasized by the process of alteration. In ordinary light caly a radial 
arrangement of the minute brown particles can be seen, but in polarized 
light the structure is further brought out by fine lines, which polarize differ- 
ently, and in which there is a good deal of secondary quartz. This radial 
arrangement often presents also the appearance of sheaf-like aggregations. 
It appears in some cases to affect what were originally crystals of feldspar, 
though it may be chiefly an alteration of the matrix. 
Nearly as abundant as these pebbles are others of a medium-grained, 
very highly crystalline granite, which, macroscopically, is seen to be chiefly 
formed of fine orthoclase and plagioclase feldspars and gray quartz. Soft- 
ened black crystals of hornblende, besides chlorite and epidote, are also 
visible. The thin section reveals a rock composed almost wholly of ortho- 
clase, less abundant oligoclase, and original water-filled quartz. The feld- 
spars are all highly clouded and pinkish-tinted. An opaque brownish sub- 
stance represents biotite and hornblende in small quantity. Some particles 
of augite, calcite and secondary quartz are seen. 
Much rarer than any of the preceding are pebbles of a true quartz- 
porphyry, showing a hard purplish matrix, with abundant large black 
quartzes and flesh-red feldspars. The thin section shows an iron-stained 
matrix, in which there is but feeble polarization in little flocks of particles, 
some small areas occurring in which no effect on the polarized light could be 
observed. Throughout the matrix are numerous minute brownish to black 
particles, varying in size from those readily seen with a low power to those 
which could barely be defined with the highest power at command. The 
quartzes are very large, and present the usual characteristics of rounded 
doubly terminated crystals and embayments of the matrix. Particles of 
