LITHOLOGY. l6/ 



applied it can only be classified as a variety of diabase. This is the 

 opinion of Mr. Rosenbusch, and other eminent lithologists. 



Fig. I on PI. lo 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 wdth 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 5 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 



