E. S. Dana — Metrography of the Sandwich Islands. 455 



feldspars of a considerable relative length, often from \ to \ of 

 the diameter of the stalactite, as seen in a longitudinal section. 

 In one case they showed a marked tendency to parallelism 

 with the axis of the stalactite, but in other cases this was less 

 distinct. A partial concentric arrangement as seen in a trans- 

 verse section was also noted. The feldspars often have black 

 longitudinal inclusions probably of magnetite, and their cross 

 sections, square or rectangular in outline, then have a large 

 black center of the same form. A rather deeply colored 

 greenish yellow augite, somewhat pleochroic, is packed in 

 among the feldspars, and occasionally shows sharp crystalline 

 outlines. There are also numerous grains and octahedrons of 

 magnetite, and throughout a multitude of beautiful dendritic 

 forms branching at angles of 90° or 

 of 60°. This is one of the most a _ &- 



marked characters of the sections. 

 The areas, where these iron dendrites 

 are crowded together, are less dis- 

 tinctly individualized, but no glass 

 was noted. Chrysolite is also absent. 

 Figures 9, a and b will convey some 

 idea of the appearance of the longi- 

 tudinal and transverse sections, 

 though the relative amount of feld- 

 spar and augite is rather too small. 

 The fact that the structure is throughout coarsely crystalline 

 with the normal constituents of the basalt — except the chryso- 

 lite — is an important point. 



The occasional cavities or open spaces in the solid parts of 

 the stalactite are often beautifully lined with large rhombic 

 tables of feldspar, perfectly clear, and so excessively thin as to 

 suggest scales of mica ; also dark needles of augite, often curved 

 and wire-like, and octahedrons of magnetite. (See 8,/".) The 

 feldspar plates have mostly the form of a symmetrical lozenge 

 (fig. 10), with angles of 128° and 52°; one side is shown by 

 the cleavage to be bounded by the 

 basal plane, the other by the dome 

 x (101). The extinction makes an 

 angle of — 7° to —9° with the basal 

 edge which conforms to that of 

 andesine, that is a plagioclase some- 

 what more acidic than that deter- 

 mined in the rock mass. These 

 feldspar plates are often marked on the edges with a thin black 

 scale, presumably magnetite, with numerous minute circular 

 open spaces containing many black points as if the whole 



Sections of lava- stalactites 

 ( x 3), a longitudinal ; b trans- 

 verse. 



Am. Jour. Sci.— Third Series, Vol. XXXVII, No. 221. 

 29 



-June, 1889. 



