AXIOLITIC EHYOLITE. 419 



In certain kinds of rhyolite, apparently composed of welded glass 

 fragments, there is a microspherulitic growth which bears a definite relation 

 to the form of the supposed glass fragments. The feldspar fibers are in 

 groups, which are approximately normal to the outline of the fragments, 

 and radiate inward. Where the fragment had a rudely triangular shape 

 the central part often attained a greater degree of crystallization than the 

 margin, and sometimes consists of distinct crystals of feldspar with tridymite 

 or quartz and a small amount of ferromagnesian mineral. Where the 

 fragments were long and narrow the spherulitic growth from the sides 

 inward produced the effect of parallel fringes — "axiolites" of Zirkel. This 

 tendency to develop spherulitic growths from old surfaces or cracks is 

 shown in another modification of the rock in which the axiolitic structure 

 can be seen megascopically. In hand specimens these varieties appear t< > 

 be dark lithoidal or glassy rocks, traversed in all directions by narrow 

 spherulitic bands. In the glassy forms of the rock the black glass inclosed 

 by the lithoidal bands occasionally falls out, like a kernel from a shell, 

 proving the spherulitic portion to be a growth along intersecting planes. 



In thin section it is observed that the spherulitic bands have a dark 

 line along their centers, as though they were ancient cracks. They inter- 

 sect one another in some cases, but are not persistent in others. Their 

 behavior toward phenocrysts when present is the same as that of perlitic 

 cracks. They encircle them, but never traverse them (PI. LV, fig. 4). 

 They do not often occur in as many concentric circles as ordinary perlitic 

 cracking, but they are unquestionably of the same general character, 

 namely, the cracking of an amorphous glassy substance. The spherulitic 

 crystallization is subsequent to the cracking and is located along the cracks. 

 It is like other forms of spherulitic growths in these glasses, and is evidently 

 only a special case. In one instance similar spherulitic g'rowth had formed 

 about the edge of an open crack in such a manner as to show that the 

 magma had been so viscous previous to its last movement that a small gap 

 in it was not closed. Upon its walls pellets of tridymite formed. It must 

 have been after it had reached this highly viscous state that this particular 

 spherulitic crystallization took place. In cases where perlitic glasses 

 contain spherulites it is observed that the perlitic cracks encircle spherulites 

 in the same manner as they encircle phenocrysts, and do not traverse them. 

 The perlitic cracking is' subsequent to the spherulitic crystallization and is 



