No. 2818. PETROGRAPHY OF DIKE ROCKS OF IDAHO—SHANNON. 489 



lamprophyres of this region in that in color it ranges from pale 

 greenish-brown to deep brownish-gi-een. The quartz forms interlock- 

 ing grains which are relatively free from inclusions. Small well 

 crystallized apatites are sparingly scattered through the rock and 

 occasional large phenocrysts of diopside occur surrounded by horn- 

 blende in parallel position. Iron ore seems to be absent. Among the 

 secondary minerals, the most important is chlorite, formed at the 

 expense of both the biotite and hornblende. Areas of infiltrated cal- 

 cite, small masses of epidote and abundant ragged grains of pyrite 

 make up the list. 



The hornblende is very pleochroic in tones of greenish brown. 

 The prisms of hornblende which are in part well bounded by crystal 

 planes show well-developed cleavage and are very frequently twinned 

 parallel to (100). The extinction angle, measured from the twin- 

 ning plane, is near 16°. This brownish hornblende is apparently a 

 variety intermediate between common and basaltic hornblende very 

 near the latter as in the other spessartites described below. In this 

 rock, however, cores of brown hornblende are frequently surrounded 

 by an outer fringe of common green hornblende in parallel position. 



This quartz-hornblende rock is very unusual in composition and its 

 mode of occurrence would suggest that the unusual composition 

 might be due to alteration of a contact-metamorphic nature incident 

 upon the intrusion of the later more feldspathic rock into the same 

 fissure. Opposed to this hypothesis, however, is the fact that the 

 altered rock equals in amount the later rock, which might have 

 caused the alteration. Furthermore, where these magmas are in con- 

 tact with the inclosing sediments, they have nowhere exerted any 

 noticeable metamorphic influence. The rock from the Hecla Mine 

 described by Ransome as consisting chiefly of hornblende with a 

 large amount of included quartz gives a reasonable explanation of the 

 pecularity of the Bailey's Pond quartz-hornblende rock. In all 

 probability it is the same as the " odinite " of the Hecla dike — a 

 magma consisting entirely of ferromagnesian minerals which de- 

 rived and absorbed a large amount of silica from the quartzitic rocks 

 through which it was intruded. This derived quartz became thor- 

 oughly diffused and recrystallized as a part of the fabric of the rock 

 instead of remaining as recognizable inclusions as in the Hecla dike. 



Bailey^s Pond, Spessartite. — The rock which is intruded into the 

 same fissure forming a mixed dike with the one described above 

 varies greatly from place to place in texture and appearance. Per- 

 haps the most abundant type is medium-grained equigranular and 

 of a dark gray color but showing, under a lens, a " pepper-and-salt " 

 aggregate of glittering black hornblende prisms and grains of feld- 

 spar. In other parts of the dike this rock is porphyritic from the 

 development of numerous phenocrysts of white feldspar averaging 



