CONSTITUTION AND ORIGIN OF SPHERULITES. 419 
has studied. On the other hand, certain facts to he brought 
out show conclusively that in very many interpretations of 
spherulitic bodies the presence of indefinite substances—crys¬ 
tallites, petrosilex, or microfelsite—has been unnecessarily 
and unjustifiably assumed under the influence of precon¬ 
ceived ideas. 
Description of Spherulites from Colorado. 
The spherulitic rocks examined by the writer come mainly 
from the Silver Cliff-Rosita mining district, in Custer county, 
Colorado, where, on the western slopes of the Wet Mountain 
range, there is a small group of hills made up of a series of 
andesites, rhyolites, and trachytes from a single volcanic cen¬ 
ter. The rhyolite was poured out through numerous fissures 
and ejected from explosive vents, the products intermingling 
in a complex manner. The rock is now found in dikes, in 
remnants of flows, and in breccia or agglomerate masses, 
almost all of which are characterized by an abundance of 
spherulitic growths. In many cases the entire mass is spher¬ 
ulitic, in a number of generations, the conditions favorable to 
such crystallization having been repeatedly interrupted, to 
be renewed with distinct, though minor, changes in the char¬ 
acter of the successive products. 
In these various rock masses spherulites occur of all sizes 
between those of microscopic dimensions and complex bodies 
more than ten feet in diameter. In form they vary from the 
perfect sphere through almost the whole series of modifica¬ 
tions so easily produced by this kind of crystalline growth. 
These multitudinous forms are visible in many relationships 
to each other, to glassy rocks, and to banded lithoidal rhyo¬ 
lite. They are sometimes found in fresh condition in pitch- 
stone, and in other cases occur in masses which have been 
subject to decomposition in several ways. The material is 
therefore adapted to show the original relationships of the 
bodies to the consolidation of the magma, different kinds of 
internal structure and mineral constitution, and also changes 
due to entirely secondary processes. 
54-Bull. Phil. Soc., Wash., Vol. 11. 
