70 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
inch and less long. It contains scattering interpositions of an opaque black sub- 
stance, and minute brown particles, which may be, or have been, glass. The crystals 
of plagioclase have predetermined the contours of all the other constituents, except 
the olivine, which crystallized first. The predominating feldspar is near anorthite, as 
determined by the angle between the principal sections. Occasional exceptionally 
large individuals, evidently cut in the plane 7, have their principal sections at an 
angle of 25°, with the edge O: 7, which would indicate albite or labradorite. The 
augite is very fresh and transparent, almost colorless in the thin section, but with a 
tendency to purple-gray. An imperfect cleavage is indicated by somewhat irregular 
parallel fractures. It fills the interstices between the closely-packed individuals of 
feldspar in such a manner that a single pyroxene crystal incloses many hundreds of 
these, while its crystalline integrity is shown by the uniform color in polarized light, 
and by the arrangement of the cleavage cracks throughout the area of the augite 
individual. It is a remarkable fact that, while these large individuals of pyroxene 
contain thousands of feldspar crystals, they inclose only very few of olivine or of mag- 
netite. These minerals, together with the unindividualized substance, are crowded 
into the spaces between the pyroxenes. In this intermediate space, which surrounds 
the pyroxene individuals with.a continuous net-work, we find, also, a few small py- 
roxenes, just as isolated grains of olivine occur in the pyroxene areas. <A careful 
examination of this occurrence will, I think, convince the observer that, at the time 
the pyroxene crystallized, both the olivine and feldspar crystals, and apparently the 
magnetite, were already individualized; for where we find any of these in contact 
with the augite we find that the latter has adapted itself to the already defined con- 
tours of the others. While the augite inclosed the feldspar erystals with ease, it 
crowded the other constituents almost wholly into the surrounding spaces—a process 
which was facilitated by the presence of the then fluid unindividualized substance. 
The magnetite is in irregular-shaped bodies, which mold themselves sharply around 
the contours of the feldspar and olivine. The olivine is abundant in grains and 
roughly-outlined crystals, but, as a rule, however fresh the melaphyr may otherwise 
be, the olivine is partly or wholly altered. 
The following is Pumpelly’s more detailed account of the internal 
changes which these luster-mottled rocks have at times undergone! The 
residuary base and the olivine are usually wholly altered, while the augite 
and plagioclase are much more commonly fresh than in the olivine-free 
kinds. 
The first and ever-present stage of alteration is caused by the change of the 
residuary magma-base which fills the interstices between the crystalline constituents, 
and in places penetrates into or is inclosed in the interior of these. The physical and 
chemical character of this seems to have predisposed it to an easy change. It is now, 
as arule, when seen in thin sections, a darker or lighter olive-green substance, and 
very soft under the needle (hardness not over 2.5). In polarized light it exhibits a 
'“Metasomatic Development of the Copper-bearing Rocks of Lake Superior.” Proc. Am. Acad. 
Sci., Vol. XIII, pp. 269-270. 
