MICEOGEAPHIC INTEEGEOWTHS IN EHYOLITE. 411 



each individual having a fibrous structure in several directions, and in places 

 a granular structure. This is shown in PL LII, fig. 3, which represents a 

 section through two individuals of feldspar with nearly rectangular outline. 

 The fibration lies almost perpendicular to the sides of the rectangles. The 

 outline of the rectangles is serrated by the projection of minute crystals. 

 The identity of such forms with intergrowths of quartz and feldspar is 

 apparent. An illustration of a large group is introduced for comparison 

 (PI. LII, fig. 5). It is similar to the graphic structure associated with 

 phenocrysts in rhyolitic pumice, already described. Such intergrowths 

 are primary crystallizations from the molten magma, and exhibit the idio- 

 morphic outline of quartz or feldspar when large enough to be recognizable. 



As the number of feldspars which combine in a group increases, the 

 outline of the cluster becomes more and more oval or circular and the form 

 of the feldspar individuals is lost. An illustration is given in PL LII, fig. 

 4, in which the feldspars wedge out toward the center, their outer ends 

 making an almost continuous outline. The fibration is in wedge-shaped 

 sets and does not radiate uniformly from the center. In this case the extinc- 

 tion of light between crossed nicols is quite irregular, as shown in the 

 illustration. This is due to the variable orientation of the feldspar crystals, 

 which have a generally radiate arrangement, and also to that of the quartz, 

 which is more irregular. Other compound growths which are analogous in 

 structure and optical behavior to the micrographic are certain kinds of 

 spherulites that occur in great profusion in most of these rhyolites. The 

 simplest and smallest of this class of spherulites are minute colorless 

 spheres, 0.2 to 0.05 mm. in diameter. They have a fibrous structure notice- 

 able in strongly convergent light, and exhibit a more or less well defined 

 dark cross between crossed nicols. The arms of these crosses change their 

 shape and split into branches near their outer ends during the rotation of the 

 section (PL LII, fig. 2; also PL LIII, fig. 1). 



In some cases a colorless fibrous margin surrounds the micrographic 

 groups; its character and length of fiber correspond to those of the 

 microspherulites just described. From its optical behavior it appears to be a 

 crystallographic continuation of the materials of the micrographic kernel. 

 These spherulites are often in rows and layers (PL LIII, fig. 1), and some- 

 times their centers are so close together along straight lines that in thin 

 section they produce the effect of transparent fibrous bands (PL LIII, 

 fig. 1). Such a band of colorless spherulites is shown in PL LV, fig. 1. It 



