88 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
from that of the rest of the bed. This difference consists principally in 
greater denseness of grain, from solidification while much of the matter 
was not developed into distinct minerals. The difference is least, then, in 
the case of those beds whose lower portions are composed of some phase 
of the diabase-porphyrites, in which there is also a greater or less proportion 
of unindividualized matter. In some of these beds, especially when the 
rock is of the dense brownish kind with highly conchoidal fracture, above 
described, there is no perceptible difference between the matrices of the 
vesicular and non-vesicular portions of the bed; but more usually there is 
a great difference in this respect between the lower and vesicular portions 
of a flow. The internal changes to which such an open vesicular substance, 
composed largely of a molecularly unstable material like glass, must always 
be liable, have greatly increased the difference, and have given rise, by the 
variation in the decomposition-products, to a great variety of amygdaloids, 
which it would seem at first sight hard to place together. 
Under the microscope the matrix of the unaltered, or relatively little 
altered, amygdaloid shows nearly always much non-polarizing matter, 
commonly deeply stained with red ferrite. In this are developed needles 
of plagioclase to a greater or less extent, and often these needles seem to 
be but microliths arrested in the process of aggregation into crystals.’ 
Augite particles occur, but are usually relatively sparse, and frequently 
fail entirely. Very often there is a fluidal structure brought out in the 
arrangement of the plagioclase microliths and other particles, and in many 
cases the flowage direction is found to coincide with the longer axes of the 
elongated vesicles. The vesicles themselves, filled or empty, as the case 
may be, are always sharply outlined in the thin section, and there is im- 
mediately about them a crowding of the plagioclases and ferrite particles, 
as if by pressure in the cavity. Moreover, the individualized minerals, as 
Pumpelly has shown,” are often more minute in the vicinity of the vesicles 
than away from them. Porphyritic feldspars, macroscopically visible, are 
frequently developed in the matrix of the amygdaloids—so far as my 
observation has gone they are at least as often present as not—and in this 
1R. Pumpelly, ‘‘Metasomatic Development,” p. 282. 
2Tbid., p. 283. 
