44 



BULLETIN OF THE 



magnetite, and viridite. • The strnctnre is that belonging to basalts, and 

 the rock is comparatively little altered considering its age. The feld- 

 spars contain inclusions that appear to have been portions of the origi- 

 nal base, but whtch, as well as the base adjacent, has been altered to 

 a grayish-white globulitic and fibrous material. This shows largely 

 the structure of the original globulitic base, but wants the black color. 

 The fibrous alteration product of the base has been confounded with 

 the true original micro-felsitic base of the andcsites by lithologists 

 generally ; as is the inevitable result of studying tlie constituents of 

 rocks without regard to the origin of these constituents. A very little 

 of the original globulitic base was ibuiid unchanged in some of the 

 feldspars. IS^umerous microlites extend through the groundmass that 

 belong apparently to apatite. This is probably from the same dike that 

 No. 1110 of Mr. Brooks's collection, described by Dr. Wichmann, was 

 obtained.* The "long, small rod-like crystals" in which the plagioclase 

 is said to occur is the usual ledge form which belongs to basaltic plagio- 

 clase. The " inserted substance, which has not been individualized," is 

 doubtless the altered globulitic base. No biotite or hematite were seen 

 in No. 320, which was selected with direct reference to procuring as 

 unaltered a specimen as possible, as well as one removed from the influ- 

 ence of the ore and jasper (342) through which the mclaphyr cuts. A 

 section showing the jimction of the two rocks (342) was made. The 

 mclaphyr here becomes a dense black opaque mass, containing a few ledge- 

 formed plagioclase crystals and some decomposed (viriditic) augite. Thia 

 part is probably an altered tachylitic glass, such as occurrs in similar 

 positions in basalts. The junction with the jasper is w^ell marked, and 

 more regular than one would expect to find it. The globulitic structure 

 shows well in the melaphyr at this immediate line of contact. The 

 *' jasper" is composed here of quartz and magnetite. The quartz is in 

 part fine granular, and filled with magnetic dust ; while the remainder 

 is coarsely granular, having the same structure as the granite (greiscn) 

 southwest of Republic Mountain (No. 128, p. 53). These quartz grains 

 were formed by the fissuring of a quartz mass, not built up of detrital 

 quartz grains, as is the case with a true quartzite according to the defi- 

 nition employed by us. The quartz contained numerous fluid and stone 

 inclusions, as well as microlites, magnetite, and lenticular scales of proba- 

 ble muscovite. 



The "diorifco" (334, 335) cutting the magnetite yielded specimens 

 showing a well-defined contact (337, 338, 339). No, 335 is a grccnish- 



* Geo!.' of Wise, in. 625. 



