K S. Dana — Petrography of the Sandwich Islands. ^Al 



litic; on this account the specific gravity is much higher, it 

 varying from 3*00 to 3 '20.* In many cases they have suf- 

 fered some alteration which has given them a dull waxy sur- 

 face, while the large grains of chrysolite are frequently iridescent 

 and sometimes have an almost metallic luster. The color varies 

 with the amount of iron oxidation from light gray to dull red- 

 dish gray or brown. The mineral constituents present are those 

 of normal basalt ; and most prominent among these is the chryso- 

 lite ; in some specimens it must make up nearly half the mass of 

 the rock ; and in one case (102) probably more, this particular 

 specimen haying the unusual specific gravity of 3 "20. The chry- 

 solite was evidently early separated from the magma, and the 

 changes of condition through which the lavas have passed is well 

 shown in the irregularly corroded or occasional broken form of 

 many of the crystals and grains. Even when there is a distinct 

 crystalline outline, it is not a rare thing to find the crystal broken 

 and the parts slightly separated. This is shown in the accom- 

 panying figures 4, a iof. Some of the corroded forms take very 

 fantastic shapes. A novel and common feature of this chryso- 

 lite is the occurrence of very slender acicular forms. The 

 length is often considerable, even when viewed macroscopically,. 

 in one case 2 to 3 mm , but in breadth they are often hardly 

 more than a line, (note fig. 4, a.) This chrysolite shows the 

 partial alteration alluded to in a broad rim of brown iron oxide ; 

 we can pass in the same slide from a crystal still preserving its 

 transparency throughout, to those where only a string of chryso- 

 lite grains mark the position of the original individual, and 

 from these to the cases where a narrow brown line of iron 

 oxide alone is left; in a few cases (as 94) the chrysolite is 

 stained bright red, showing that there has been oxidation of the 

 iron without hydration. 



The orientation of these peculiar rod-like forms, which are 

 distinctly visible on a polished surface of the rock, is a matter 

 of some interest. The fact that, in such a form as that of fig. 

 4, h, and others like it, the plane of the optic axes is transverse 

 to the longitudinal direction and the bisectrix normal to the 

 surface presented, shows that they are elongated in the direction 

 of the vertical axis, the narrow dimension being that of the 

 macrodiagonal. This chrysolite has often an unusually deep 

 green color possibly connected with the partial alteration, and 

 then shows distinct pleochroism with the absorption least in 

 the direction of the vertical axis. It often shows spherical 

 inclusions of a pale brown glass, sometimes arranged in parallel 

 lines. 



* Some of the separate determinations gave : 3-09,3-18, 3-09, 3-04, 3-00, 3-20, 

 3-00. 



