414 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



group or minute spherulite at the center. Between crossed nicols the 

 dark arms seldom form a distinct cross, but split into many arms, some 

 making an angle of 45° with the principal sections of the nicols (PI. LIL, 

 fig. 1). 



These larger spherulites are generally traversed by streams of trichites 

 and minute spherulites which continue the flow lines of the rock through 

 the spherulite without change of direction (PL LV, fig. 2). But in some 

 cases these interpositions have been crowded into radial and circular lines 

 (PI. LV, figs. 3 and 4, and PI. LVI, fig. 1). These produce the bands or 

 zones of color in the small spherulites. Often the trichites and grains are 

 red by incident light, as though the magnetic oxide had been changed to 

 ferric oxide, and it is noticeable that the black trichites and nearly colorless 

 microlites in the glass as they pass into spherulites become red, as though 

 they had encountered an oxidizing agent not active in the surrounding glass. 



The shapes of spherulitic growths are not limited to spheres, but may 

 be hemispheres, disks, and sectors, or pliime-like (PI. LVI, fig. 1), or in a 

 brush like a fox's tail. They may also appear in thin section in the form 

 of fibrous bands or fringes with all possible curvatures. All the spherulitic 

 growths just described may be referred to stellate groups of prisms of 

 orthoclase elongated parallel to the clinoaxis, and intergrown with quartz 

 whose orientation is not known to be fixed with respect to the feldspar 

 prisms. 



There are other kinds of spherulitic structures in these lavas which 

 have crystallized in a different manner. Their nature may be understood 

 from a consideration of certain forms of microlitic growth assumed by 

 orthoclase in these magmas. 



Prisms or needles of feldspars sometimes occur associated together in 

 nearly parallel groups, or in more or less divergent ones. In some cases 

 they are separated from one another by glassy groundmass; in other 

 cases by a colorless, almost isotropic mineral, probably tridymite, as will 

 be shown subsequently. These needles are straight or curved, and some- 

 times have a rough, jagged outline, sometimes a smooth one. Long* delicate 

 feldspar needles, the spaces between which are filled with tridymite, are 

 shown in PI. LVI, fig. 2. The rock also carries dark-brown spherulites, 

 which are nearly opaque in the figure. The cross sections are generally 

 six-sided or rhombic, with the acute angle truncated by a plane more largely 



