DETAILED DESCRIPTION OF SLIDES. 123 



bleude-andesite area, has a microlitic groundmass, and shows considerable 

 hornblende, I have regarded the excess of augite as local. The slide is 

 remarkable for the tact that much of the strongly dichroitic augite is sur- 

 rounded by a black border quite as broad as that which ordinarily occurs 

 about andesitic hornblendes, though not so broad as that accompanying the 

 hornblendes in this specimen. This slide contains unquestionable ilmenite 

 with rhomboidal cleavage marked by translucent lines. One of these is 

 shown in Fig. 19, Plate III. One of the masses of ilmenite incloses a twin 

 augite crystal, just as the same mineral so commonly includes apatite. The 

 apatites are mostly deep brown and dusty. 



Slide 450. 1,000 feet east of station at junctiou of Silver City Railroad. 



Specimen showing hornblende with double black border. This rOck is Ouly eXpOSCd by 



the railroad cut for a few yards, and undoubtedly underlies the adjoining 

 augite-andesite. It is dark purplish gray, and contains a very large amount 

 of visible hornblende. The slide is chiefly remai'kable for the light which 

 it throws on the character of the black border. The hornblendes are devel- 

 oped with unusual symmetry, but many of the crystals have been broken, and 

 all the fragments are surrounded by black borders. In one case a beau- 

 tifully fresh, highly dichroitic, dark brown hornblende fragment shows not 

 only a black border but a parallel band of magnetite at some distance from 

 the edge — a zonal structure marked by an interior black belt. This crystal 

 is shown in Fig. 17, Plate III. Other of the large hornblendes in the slide 

 .show the same phenomenon, though imperfectly; but the small crystals 

 have but a single border.^ 



The specimen contains considei'able augite, and the groundmass shows 

 fluidal structure, as well as the peculiar felt-like texture so common in augite- 

 andesites. It is possibly not a hornblende-andesite, in spite of the great 

 predominance of hornblende, but an augite-andesite with a local segre- 

 gation of hornblende. No other hornblende-andesite occurs for a long 

 distance, and a glance at the map will show the im23robability of any con- 

 siderable amount of that rock being entirely covered by the limited areas 

 of augite-andesite. 



' For some speculations on this occurrence see page 59. 



