CONSTITUTION AND ORIGIN OF SPHERULITES. 425 
regular outer form, and as the action of this period seems to 
have been very energetic, complex spherulites several inches 
in diameter have frequently resulted. In such a spherulite 
the early forms are simply surrounded as so many foreign 
bodies, and the type may be termed the enveloping spherulite. 
Such bodies are comparatively coarse in texture and have 
practically the same characteristics as the more limited re¬ 
sidual growths. 
What may be termed compound spherulites result from a 
regular orientation of successive growths. A most remark¬ 
able instance of this type is found at Silver Cliff, where a 
large lava flow several hundred feet in thickness was poured 
into a lake basin. The lower part is pitchstone (see analysis 
I above), -the upper, banded lithoidal rhyolite, and between 
the two is a zone, locally fifty feet thick, largely made up of 
gigantic compound spherulites. These bodies vary from a 
few inches to more than ten feet in diameter. They are 
built up by two or three successive growths about a common 
center and in several ways. In the largest ones the first 
growth took the form of very dense finely fibrous arms 
branching as they extended out from the center, often for a 
distance of several feet. After an interval the matter between 
these arms crystallized in a coarser growth, the feldspars of 
which are oriented in accordance with the fibers of the 
earlier development. This crystallization does not extend 
far beyond the limits of the first, but an outer zone is fre- * 
quently present of a third type, which may in itself build up 
certain other compound spherulites. Fig. 1, Plate 6, repre¬ 
sents the structure of the compound type, to which reference 
will again be made in considering conditions of spherulitic 
development. 
Discussion of the Mineralogical Constitution of 
Spherulites. 
It has been shown many times that the larger and more 
important spherulites of acid lavas have practically the same 
chemical composition as the rock in which they lie. This 
