342 GEOLOGY OF THE EUREKA DISTRICT. 



prated and feather-like. This structure is noticeable in ordinary light from the fact 

 that the quartz remains pellucid after the feldspar has become clouded by partial 

 alteration. Between crossed nicols, however, the appearance is very distinct. In the 

 coarser grained varieties, especially at the spot represented in Fig. 1, PL vi, the 

 field is covered with blocks of similar geometric and cuneiform figures parallel- 

 ograms, trapezoids, and variously shaped triangles the sides of all those forming any 

 single group being respectively parallel, besides which are long and narrow parallel 

 strips, two sets of which, meeting obliquely, produce a feather like appearance. It is 

 further seen that all the figures in any one group extinguish light in the same azimuth 

 or have the same optical orientation, and that the inclosing block is a single individual 

 with a different orientation, the one being quartz and the other feldspar. In this par- 

 ticular case the small figures are of quartz and throughout the field have the same 

 extinction as a central grain, the inclosing blocks being of differently oriented feld- 

 spar. This tendency to crystallize around grains of quartz or feldspar is more 

 noticeable in the finer grained varieties, where the nucleus is incrusted with a shell 

 that in section appears as a frame-like border, having a radiating structure composed 

 of variously oriented sectors, though the portions formed by the same mineral as the 

 nucleus have their axes of elasticity parallel throughout. Of the phenocrysts, quartz 

 seems to be the only one around which this special crystallization takes place. 1 

 A very fine example of this structure is found in thin section 28, where it is seen 

 to have formed after the primary crystallization of the phenocrysts, but previous to 

 the final consolidation of the micrograuitic groundmass. This distinctive structure, 

 which is characteristic of many European granite-porphyries, has been described 

 by Eosenbusch and called by him "Granophyr." 2 It is the "structure pegmatoide" 

 of the French petrographers, and is becoming generally termed micropeginatitic. 



Having described the characters of the minerals composing this granite- 

 porphyry, it remains to notice their structural combination, whose variety is the strik- 

 ing feature of this occurrence. Thin section 10, from the large area north of Wood 

 Cone, is of a porphyritic granite, with little groundmass of fine grained granitoid 

 structure. The large phenocrysts have no crystallographic outline, but pass by 

 increasing abundance of inclusions into the groundmass, which contains biotite, 

 hornblende, titanite and titanic iron, apatite, zircon, and allanite. Thin section 11, 

 from southwest of the Wood Cone, is a local modification of slight importance; it is 

 a fine grained mass without phenocrysts, with granitoid structure and composed of the 

 same minerals as the previous section, but with a greater percentage of biotite and 

 hornblende. Thin section 12, from the bottom of a gulch on the east side of the dike 

 and north of Spring Valley road, is of porphyritic, coarse grained granite. The 

 larger phenocrysts of feldspar have more or less well defined outlines. The orthoclases 



'This constitutes the quartz aureole of French petrographers. 



! H. Rosenbusch. Die Steiger Schiefer, etc., pp. 347, 352, Strassburg, 1877. Mikroskopisehe Physi- 

 ograpie. p. 31, Stuttgart, 1877. Mikroskopisehe Physiograpie, vol. n, p. 383, Stuttgart, 1886. 



