428 
CROSS. 
and the diverging twigs are in some cases apparently parallel 
to the vertical axes of the two halves of the twin. A beautiful 
instance of this forking of negative feldspar fibers occurring 
in a spherulite of Obsidian Cliff is described and illustrated 
by Mr. Iddings in his article in this volume on spherulitic 
crystallization. It may be added that Iddings has found 
positive feldspar prisms characteristic of many spherulites 
in the Yellowstone Park. 
The first and most important result of this error, in re¬ 
garding all positive fibers of spherulites as quartz, has been 
a fundamental misconception as to the character of many 
spherulitic bodies, so that their true relationship to the rock 
masses of which they are a part has not been appreciated. 
The observations made upon the spherulites of the Rosita 
Hills show that the amorphous base holding the feldspar 
needles is mainly hydrous silica. It is a curious fact that 
Michel-Levy, of all petrographers, has emphasized the im¬ 
portance of colloidal silica in influencing spherulitic growth ; 
but so long as the positive fibers of spherulites were regarded 
as quartz the amorphous material between them had to be 
considered as a kind of feldspathic paste, with a probable 
excess of silica. By regarding the needles as the silicate 
and the base as colloidal silica, the importance of the latter 
is by no means diminished, and it is made to appear in a 
natural role. Many references to petrosilex as a partially 
amorphous base impregnated with quartz or chalcedony, 
such as that cited on page 416, seem to be fully explained 
by these considerations. 
The delicate branching trichites described above as char¬ 
acteristic of certain growths are beyond question primary. 
They correspond to magnetite in some growths and to tri¬ 
chites of other forms in residual or enveloping spherulites. 
It is plain from the literature of the subject that the small 
amount of iron oxide usually present in spherulites has 
greatly obscured certain occurrences, and the manner in 
which it has been accomplished seems clearly indicated in 
some partially decomposed rocks of the Rosita Hills. 
