OLIVINITIC DIABASE—MELAPHYR. 69 
above general title. The term melaphyr is, however, used in the detailed 
descriptions of subsequent chapters whenever the residuary base appeared 
to be present. 
Although grading through coarser kinds into the coarse olivine-gab- 
bros, the fine-grained rocks here considered deserve a place by themselves. 
The gradation into the coarser kinds has never been observed in any one 
bed, and they are very strongly marked by their external characteristics 
both in the fresh and altered states. They have commonly undergone great 
alteration, the amount of change lessening rapidly as the rock becomes 
coarser. Asa type of the fresh state of this rock Pumpelly has selected 
“The Greenstone” of Keweenaw Point, which is one of the transition 
phases toward the coarse gabbros. The following is his description of these 
rocks as developed on the South Shore.’ It applies equally well to the olivi- 
nitic kinds of the North Shore, where ‘‘luster-mottled” fine-grained rocks are 
very abundant. 
In its fresh state it is dark green, or greenish black, finely crystalline, very com- 
pact, hard and brittle, and breaks with an uneven to semi-conchoidal fracture. The 
powder of the rock yields to the magnet a beard of magnetite. The specific gravity is 
2.90 to 2.95. Itis an important characteristic of this rock that its freshly fractured 
surface is mainly occupied by spots one-sixteenth to three-fourths of an inch in diam- 
eter, each of which reflects the light with a satin-like sheen. The reflection is not 
carried to the eye from all the spots at once. It is generally necessary to change the 
position of the specimen many times to observe the different reflections. Aside from 
this sheen there is nothing, either in difference of color or texture, visible to the naked 
eye to betray the presence of these spots, which might be called luster-mottlings. To 
the naked eye, this phenomenon suggests, at once, interrupted cleavage of large indi- 
viduals of one of the constituents, as the cause; but under a strong hand glass these 
reflecting surfaces show the same granular texture and character as the rest of the rock ; 
and it is only when examined under the microscope, with an objective of low power 
and in polarized light, that the appearance to the unaided eye is corroborated. We 
here find the cause in the fact that each spot is the cross-fracture or cleavage of a 
erystal of pyroxene, which, in crystallizing, has inclosed hundreds of feldspar crystals. 
The weathered surface is rusty gray, scarcely one-fiftieth of an inch thick; but it is 
covered with knobs, which are due to the more rapid destruction of the materials be- 
tween the pyroxene individuals. Examining thin sections under the microscope, we 
find the constituents to be plagioclase, pyroxene, olivine, and the alteration-product of 
the latter, as well as magnetite, and an unindividualized substance, both fresh and 
altered, occupying interstices. In thin sections the plagioclase is seen to exist in 
very sharply defined, and fresh, thin, tabular crystals, .001 to .002 inch thick, and .01 
1 Geology of Wisconsin,. Vol. III, p. 33. 
