CONSTITUTION AND ORIGIN OF SPHERULITES. 431 
of quartz may presumably sink to nothing, and in fact in one 
of the Silver Cliff rocks there are spherulites made up of in¬ 
terpenetrating crystals of apparently pure feldspar. Other 
rock constituents are but accidentally included in spheru¬ 
lites of this type. 
The second class of spherulites to be considered differs 
widely from the first in several essential characters. In the 
first place, they represent the consolidation of the whole 
magma for a certain space, excepting only what had crys¬ 
tallized out in the early microlites and phenocrysts. The 
nature of the crystallization which produced them allows of 
large dimensions and various modifications of the spherical 
form. Under this class are two primary divisions, the one 
characterized by radiating branching feldspar crystals, the 
other by a more marked concentric structure, though the 
two are connected by all manner of intermediate stages. 
Concerning this important class of spherulites the study of 
the material from Colorado has brought out or emphasized 
a number of points to be discussed. Let us first consider 
the arborescent growth of the feldspathic constituent. 
It is well known that many substances crystallize out of 
solutions in curved and branching forms, and a host of 
observations upon the synthetic products of chemical labora¬ 
tories has made clear certain of the conditions essential to 
this growth. It is plainly a study in molecular physics, and 
has been so treated by Dr. 0. Lehmann,* among others, who 
has also observed and discussed the relationships between 
these artificial products and the similar bodies met with in 
glassy rocks. Briefly stated, the cause of the curving of a 
growing needle or trichite is supposed by Lehmann to be a 
tension unequally distributed over the outer surface. If the 
tension increases the needle may break or simply crack, and 
in the latter case a forking of the stem is a frequently observed 
* Lehmann (O.). Molekular Physik. 2 vols. 8°. Leipzig, 1888. In 
this work are described and illustrated many branching crystallizations, 
with data concerning the conditions under which they were formed. See 
vol. I, pp. 378-390. 
