OLIVINITIC DIABASE—MELAPHYR. val 
fibrous aggregate polarization, and shows well its structure, which is short, fibrous, 
converging towards the center. The central portion shows sometimes little or no 
double refraction, but more generally it is filled with very minute polarizing spheres 
formed of radiating fibers. With one nicol this substance shows only absorption for 
intensity. The contours are generally sharply defined by the feldspar and pyroxene 
crystals, and the result is usually a more or less wedge-shaped form. 
The next step has been the change of the chrysolite. In the so-called greenstone 
this has been only partial; but generally in the chrysolite-bearing beds it is complete. 
The result in thin sections is a faintly green substance, soft under the needle, and sur- 
rounded, within the original contours of the crystal, by a more or less opaque deposit 
of iron oxide, which also traversed it in fissures. The green substance shows by a 
well-defined cleavage in one direction that it isin thin lamine. Between crossed nicols 
these lamin have an appearance of twin structure, polarizing the light in alternate 
lines of brilliant red and green. The whole pseudomorph becomes dark when the 
cleavage is parallel to a nicol plane; and some individuals, probably cut parallel to the 
cleavage, remain dark through a revolution of the stage. The substance is, therefore, 
very probably uniaxial. It has very appreciable absorption for intensity and a very 
feeble one for color. 
* * * * * * * 
The green substances which I have described so unsatisfactorily as being undoubt- 
edly pseudomorphous after residuary-base, pyroxene, and plagioclase, have in them- 
selves no physical properties, recognizable under the microscope, which are sufficiently 
persistent to invariably characterize them respectively. Often the best means of dis- 
tinguishing between them is in the internal structure of the aggregate, since this is 
intimately connected with certain physical characteristics of the original mineral which 
predetermined the mode of growth and gradual arrangement of the secondary product. 
In other instances, the presence or absence of cracks stained with iron oxide are char- 
acteristic. 
Thus the change of the residuary base has resulted in a tendency to form bands 
parallel to the contour of the area in a manner that indicates a gradual progress along 
concentric shells from the circumference inward or the reverse; while in the feldspar 
the growth appears to have begun without any regularity throughout the cleavage and 
twinning planes of the crystals. The pseudomorphs after pyroxene, are almost invari- 
ably defined by the red-stained cracks and slight mixtures of brown and green. 
The “luster-mottling” described by Pumpelly as due to the crowding 
of the magnetite and olivine into the interspaces of the augites is so pro- 
nounced and constant a characteristic of these rocks that, even when they 
are too fine and too much altered to show it in the hand specimen, a single 
glance at the thin section with the unaided eye is always sufficient to 
reveal it. 
As typical instances of these fine-grained olivinitic rocks may be men- 
tioned “The Greenstone” of Keweenaw Point; the rock of the north shore 
