FAYALITE IN CERTAIN IGNEOUS ROCKS 561 



ferrous silicate mineral into the oxide of iron and oxide of silica, the 

 silica being carried away in solution, probably as hydroxide of silica, 

 and the oxide of iron, which is apparently always magnetic, left as 

 residual material. Quartz as an undoubted residual product of the 

 alteration was not observed in any case. The alteration of the fayal- 

 ite to magnetite, as indicated in the photomicrograph, takes place 

 about the borders of the crystals, and along the fractures and cleavage. 



Not all the iron oxide associated with the fayalite, however, is of 

 secondary origin. There can be little doubt that some of the iron 

 oxide occurring as rims surrounding the fayalite, as well as that occur- 

 ring as inclusions within the fayalite, is of primary origin. The dis- 

 tribution of the iron oxide, probably mainly magnetite, is such as to 

 indicate that most of it occurring in the rocks is an original separa- 

 tion from the magma, for it is by no means entirely confined to the 

 vicinity of the fayalite, or of the other dark-colored constituents, but 

 often appears to be entirely independent of them. (See Figs. 1-3.) 

 Rims of hedenbergite also surround the fayalite (Fig. t,, d), and some- 

 times the reverse is true and fayalite and iron oxide nearly completely 

 surround hedenbergite (Fig. 3, a). Furthermore, these associations 

 are just as abundant in perfectly fresh rocks, as illustrated in phases 

 from a well taken fifty or sixty feet from the surface, as in specimens 

 collected from the rapids of the Wisconsin River at Wausau, and 

 elsewhere, immediately beneath the soil. 



While much of the fayalite appeared to show more or less altera- 

 tion, still in most cases the alteration was never complete, at least 

 in crystals of the ordinary, or average, size, even when the fayalite- 

 bearing rock was immediately associated with the soil, or running 

 water. Attention has already been called to the alteration of the 

 large nodules of fayahte in pegmatite, sixty feet below the surface, 

 at Rockport, Mass., where the magnetite was observed as forming a 

 shell about the fayalite. In all other cases noted in literature, the 

 alteration of fayalite has been quite generally observed as being to 

 magnetic material, and in this regard it is quite similar to the altera- 

 tion of olivine, in which case the secondary products are usually 

 serpentine and magnetite. 



Samuel Weidman. 



Madison, Wis. 



