LITHOLOGY. 167 
applied it can only be classified as a variety of diabase. This is the 
opinion of Mr. Rosenbusch, and other eminent lithologists. 
Fig. 1 on Pl. ro represents a magnified section of this rock as it ap- 
pears in polarized light. The diallage possesses peculiar outlines, owing 
to the influence of the feldspar upon it. These crystals crowd upon it, 
often pierce through its margin, and sometimes a rectangular feldspar 
crystal is wholly enclosed in the diallage. Hypersthene is recognized 
by its orthorhombic behavior in polarized light, and by its peculiar inter- 
positions, which are arranged in three definite planes (see p. 54). The 
olivine is yellowish-green, and in thin sections it is light yellow, and being 
fresh and undecomposed it gives very brilliant interference colors when 
examined with polarized light. As usual, it is traversed by irregular 
rifts, which are made very prominent by the black stains caused by 
beginning decomposition. It is often impure on account of the enclosed 
magnetite. The black grains of iron oxide are very abundant in this 
rock. It is not crystallized, and in part at least is very magnetic; and 
as it has been shown by Mr. Dana that it is quite titanic, it is a titanic 
magnetite. In some grains of this magnetite I once found some little 
specks of metallic iron. Dr. J. Lawrence Smith, to whom I gave some of 
the rock, also found some; but it is not easy to find it when one seeks for 
it, and it may have resulted from the accidental reduction by some car- 
bonaceous material that came in contact with the oxide when it was hot, 
and may be very local, and not widely distributed through the rock. If 
this is so, it has no special lithological significance. 
The labradorite is very white and clear in thin sections, although it is 
filled with impurities. It contains numerous grains of augite and magne- 
tite, scales of biotite, and crystals of apatite, and sometimes innumerable 
minute needles run in several well defined directions through it. These 
needles are common in the labradorite of gabbros. They were described 
on page 94, and Fig. 5 on Pl. § represents them. The labradorite in 
polarized light is banded with the most brilliant colors; but, as indicated 
in the figure, the exact parallelism of the bands does not extend over any 
great width of the grains. 
The apatite is microscopic, but some of its crystals are quite large. 
They exhibit most interesting peculiarities. Sometimes little crystals, 
or crystalline forms of darker color, are arranged in their interiors, with 
