84 THE CRYSTAL FALLS IRON-BEAliING DISTRICT. 



as to exhibit the difference in relief of tlie minerals, and, best of all, between' 

 crossed nicols. (See fig. A, PI. XX.) The zones are seen to be made up 

 of reticulating- areas of clear quartz, in which lie embedded in-egular pieces 

 of feldspar. Where two or more of the quartz stringers or needles unite, 

 one sees broad areas of limpid quartz. The network of quartz is best seen 

 when it exhibits its highest polarization color, as then the feldspar is for the 

 most part dark. The pieces of feldspar in such a quartz area for the most 

 part have irregular orientation, as is shown by their varying extinction, 

 although a number extinguish simultaneously. This quartz net is connected 

 with the quartz phenocrysts, as shown by the continuation of the qiiartz of 

 the phenocrysts and that of the zone, and the consequent agreement in 

 orientation. The lack of a uniform optical orientation of the feldspar grains 

 is made especially apparent when the quartz is cut perpendicular to the c 

 axis, and consequently remains dark under crossed nicols. Under the 

 above circumstances we see certain feldspar grains polarizing in the zone 

 around the quartz, and as the stage revolves other particles lighten as those 

 which polarized in the previous position of the stage become dark. From 

 this description it is evident that the texture is not micropegmatitic according 

 to the generally accepted definition of the term, but coiTesponds to the 

 micropoikilitic, as described by Iddings and Williams.^ 



A gradation toward a spherulitic texture was noticed in one instance 

 where a number of long quartz stringers were arranged perpendicular to 

 the periphery of the quartz phenocryst. (Fig. B, PL XX.) The texture 

 of this micropoikilitic mass, it will be observed, is finer than that before 

 described. 



The groundmass of the porphyries is formed of irregular roundish 

 areas having exactly the same micropoikilitic texture as the zones surround- 

 ing the quartz phenocrysts. An explanation of the origin of the zones 

 should therefore also explain the texture of the groundmass. Certainly in 

 many cases, probably in most cases, the groundmass areas result from 

 tangential sections through one of the micropoikilitic zones surrounding 

 the quartz phenocrysts. 



The descriptioii given by Hedstrom^ of this same structure as observed 

 by him is essentially the same as the above, if I have understood him cor- 

 rectly. The following difference is, however, to be noted. In speaking of 



' Op. cit., pp. 589 and 179. ^Qp. cit., p. 8. 



