NOTES UPON THE HENRY MOUNTAIN BOCKS. 1 



The Henry Mountain rocks are of two principal classes, one horublendic, the 

 other angitic, with plagioclase as the predominant feldspar in both cases. 



HOENBLENDIC ROCKS. 



Macroscopical The horublendic varieties have as a class a much more recent ap- 

 pearance than theMosquitoKangeporphyrites r with which they agree in composition and 

 microscopical structure. This arises from theprevailing light-grayish tone of the ground- 

 mass and the glassy luster of the feldspars. Nearly all specimens show a decidedly 

 porphyritic structure, although they vary greatly in the relative proportions of the 

 groundmass to macroscopic elements. A white or glassy, colorless feldspar, in short, 

 stout crystals, or less frequently in tablets, and a glistening dark hornblende are the 

 only macroscopic minerals of prominence. A few rounded quartz grains are visible 

 in some of the specimens, and pale-yellow, brilliant crystals of titauite can be detected 

 in most of them; also, minute ore grains. The gronndmass in which these minerals 

 lie is gray or tinged with red when fresh, but is greenish or dull gray when attacked. 

 Careful search with the lens shows the characteristic striatiou of triclinic feldspars on 

 many individuals, but it is much less prominent than usual. The hornblende is sub- 

 ordinate both in size and number of its crystals, and seldom appears in the ground- 

 mass in sufficient quantity to give it a greenish tinge, as was common in the Mosquito 

 Range porphyrites. Few feldspars reach a diameter greater than l cm , while the 

 average is below 0.5 cm . 



Microscopical No. 04 will be first described, as it corresponds so nearly to our 

 Mosquito gulch type, and the mutual relations of the two rock groups can thus be made 

 most easily apparent. The only minerals appearing in large crystals are feldspar and 

 hornblende. No quartz grains fall in the section. Other minerals to be distinguished 

 from those in the groundmass are zircon, apatite, magnetite, and possibly some titanic 

 iron. An unknown pale-green mineral, polarizing strongly, is present in irregular 

 grains in the groundmass (pyroxene?). It is not abundant. 



The feldspar is clearly plagioclase in nearly all cases when seen in polarized 

 light. Most crystals show distinct laminse running fully across them, but others con- 



1 These notes were prepared at the request of Mr. Emmons for purposes of comparison with the 

 eruptive rocks of the Mosquito Range. The examinations were made upon small specimens and thick 

 sections, comparatively few new sections having been made. As the material was in a measure incom- 

 plete and is no longer at hand for further study, the notes are presented without elaboration iu sub- 

 stantially their original form. The references are to the notes of Capt. C. E. Dutton in G. K. Gilbert's 

 report upon the Henry Mountains. 



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