l68 MINERALOGY AND LITHOLOGY. 



their axes and sides parallel to the crystal that encloses them. Some 

 crystals are filled with cavities which are apparently empty ; and the 

 sides of many of the crystals are eaten through as if by some reagent. 

 Sometimes only the margin is attacked, and sometimes three quarters of 

 the crystal is eaten away. Some of these crystals are represented in 

 Fig. 6 on PI. 8. The apatite was the first mineral to crystallize in the 

 rocks, since its position was apparently taken independently of the other 

 minerals. These etched crystals are explained on the supposition that 

 after the apatite had crystallized from the cooling mass there was a 

 return to former conditions, whereby the apatite was again partially de- 

 stroyed. The perfect crystals may have formed subsequently. 



Of accessory constituents, biotite is the most common. It is very 

 dark in color, and very dichroic. A grain of sphene is occasionally met 

 with. Serpentine and chlorite are sometimes present as decomposition 

 products in superficial specimens. 



One very peculiar rock forms a bed of some magnitude in the neigh- 

 borhood of the Waterville gabbro. It is a coarse-grained rock, light in 

 color, and resembles diorite. Its feldspar was analyzed by Mr. Dana, and 

 shown to be labradorite, but all the minerals that form the rock are much 

 altered. When thin sections are cut from this rock, all the ingredients of 

 the gabbro are found as cores of the decomposition products, with the 

 exception of the olivine, which is more easily altered, and which is com- 

 pletely changed into green serpentine. The unaltered remnants are 

 identical with the minerals of the rock just described; and the very 

 peculiar and striking appearance of this rock is due to the strong con- 

 trast into which the minerals are brought by decay. The originally black 

 labradorite has become opaque white; the black diallage is light brown, 

 or green ; but the magnetite maintains its old form and lustre, and the 

 apatite is also intact. The presence in the apatite of its very marked 

 peculiarities, which render it so interesting, points conclusively to the 

 circumstance that this is only a form of gabbro which has resulted from 

 decomposition. 



The masses of gabbro that occur on the Mt. Washington river present 

 only minor variations. The rock occurs in immense masses that are best 

 exposed on the borders of the river-bed. It is lighter in color than the 

 Waterville rock, because it contains more labradorite and less magnetite. 



