:j,S,S GEOLOGY OF THE EUREKA DISTRICT. 



various clirectious; frequently curved, presenting the appearance common to pearlite 

 structure, a beautiful example of which is to be seen in Fig. 11, PI. iii. Inclu- 

 sions are veiy rare, the only kind noticed being of colorless glass with a fixed gas 

 bubble. The olivine is more or less decomposed in every instance, the alteration pro- 

 ceeding in two different ways, which are not found in association in the same thin 

 section. One is the characteristic alteration into serpentine, in which a green fibrous 

 aggregate is formed, the fibers projecting normally from the fractures, iu which is 

 Irequently deposited iron oxide. The resulting product has the appearance of a 

 network with meshes of variously oriented fibers,,which are at times so intimately 

 mixed as to produce aggregate-polarization. In thin section 292 the color is green, 

 with only a small amount of reddish yellow; but in thin section 286 its color is 

 brownish green. The other kind of alteration may be seen in thin sections 282, 

 284^, 269, and also in 295, 296, and takes place in a different manner. There com- 

 mences from the surface and fractures as in the ordinary process a fibration, not 

 in directions always normal to the surfaces of fracture, but in lines parallel through- 

 out the entire crystal, and parallel also to some direction in the plane of the more 

 perfect cleavage. The fibers have a light yellow color at first, which deepens 

 into a reddish brown or blood red as the decomposition proceeds; they polarize 

 light brilliantly and show a parallel extinction and sometimes a faint pleochroism. 

 Ill some cases there appear reddish yellow scales and thin plates and a general 

 lamination, and less frequently the lamination or fibration is altogether wanting, 

 when the section yields a, nearly uniaxial, negative iuterference figure, the plane of 

 the optic axes in the other cases being fouiul to be perpendicular to the direction of 

 the fibers. The alteration in some individuals has started from the center, leaving 

 the outer portion still fresh. It is represented by Figs. 11, 12, 13, PI. iii. The ordi- 

 nary serpentine alteration product is sometimes colored the same orange or blood red, 

 but is easily distinguished by its internal structure, which is that of irregularly 

 aggregated fibers, not of uniformly parallel fibers. A distinction between the two 

 has not been made by Prof. Zirkel, for in his Basaltgesteine he describes a reddish 

 serpentinization of olivine from the basalt of Kotzhardt in the Eifel, and afterwards 

 a form of decomposition of the olivine in the basalt from Steinheim near Hanau, that 

 corresponds exactly to the second process, just described, and says in conclusion that 

 it is still doubtful whether the " reddish yellow " originates immediately from the fresh 

 mineral or first from the "green."' And again in his report on the microscopical 

 petrography for the Exploration of the Fortieth Parallel he remarks that, in the excel- 

 lent basalt from east of Spanish Spring Station, in the Virginia Range, "olivine 

 occurs, its larger crystals altered along the borders and cracks, and its smaller ones 

 filled with a brownish red, somewhat fibrous substance, which is, without doubt, of a 

 serpen tinous character."^ The thin section of this rock has been examined and the 



'F. Zirlccl. Ba-saltgesteine. Bonn, 1870, p. 65. 



»F. Zirliel. ilicroscopical Petrography. Wa.shington, 1876, p. 230. 



