426 
CROSS. 
composition renders it inevitable that the spherulite, if holo- 
crystalline, will contain alkali feldspar as its chief constituent 
and free silica in some form as the only other quantitatively 
important element; yet many spherulites, seemingly like 
some forms developed in lavas of the Rosita Hills, have been 
described as quartz spherulites, without reference to the 
implied absence of feldspar, which would certainly be remark¬ 
able. Such descriptions are often, if not always, based upon 
the erroneous assumption that optically positive fibers or 
needles of spherulites cannot be feldspar, and are therefore 
probably quartz. This assumption seems to be merely an 
unwarranted extension of the rule, now commonly recognized, 
that free microlites of alkali feldspar in vitreous acid rocks 
are characteristically elongated parallel to the clinoaxis, and 
thus have the negative optical character of the prismatic 
zone. It has been found true of the spherulites above de¬ 
scribed that their feldspar needles are in some cases negative, 
and in others positive, in character. This observation, if 
substantiated, casts doubt upon all determinations of quartz 
needles in spherulites, which rest alone upon the optical 
character displayed, and throws light upon the origin of 
some important misconceptions in regard to spherulites. 
As far as the coarser crystalline spherulites of the Rosita 
Hills are concerned, the determination of the branching 
needles as feldspar is justified by the observations that the 
extinction is not always parallel to the length axis, but 
varies from 0° to more than 15°, and that cross-sections show 
the mineral to be biaxial, while what appears to be a normal 
quantity of free silica is developed in quartz grains or tridy- 
mite scales between fibers. By using the quartz plate a teinte 
sensible in the well-known manner, the application of which 
to the study of spherulites has been specially recommended 
by Michel-Levy in “ Les Mineraux des roches,” the greater 
number of feldspar fibers are found to be optically positive 
in character, while a lesser proportion are negative. In the 
hollow, the trichitic, and the supplemental spherulitic 
growths the feldspar fibers are all positive; in the dense 
