418 
CROSS. 
sively the product of European writers. The only impor¬ 
tant contribution to the subject which has been made by an 
American is contained in the description by J. P. Iddings * 
of the wonderful material of Osidian Cliff, in the Yellowstone 
National Park. In beauty, freshness, variety, and degree of 
development, the spherulitic rhyolite of this locality seems 
to surpass any previously described occurrence in the world. 
Spherulites of several types are here associated, from ex¬ 
tremely minute microscopic forms to those several inches in 
diameter, and the relationship between solid spherulites and 
the hollow variety termed lithophysse is very plainly shown. 
The publications of Iddings regarding this excellent ma¬ 
terial contrast very markedly with those of European writers, 
in that he does not mention in his description of any variety 
of spherulites a crystallitic or otherwise indefinite substance, 
such as microfelsite or petrosilex. He interprets all spheru¬ 
lites studied by him as made up of known minerals, which 
are not different in kind from the constituents of the rhyolite 
formed from the same magma under other conditions. In 
other words, he treats these spherulites as produced by the 
crystallization of a magma under special conditions causing 
the peculiar structure, and not as mere consolidation 
products, caught in a certain transition stage between the 
molten fluid and the normal crystalline state. As shown by 
mineral and chemical composition and geological relation¬ 
ships these spherulitic masses are simply primary structural 
modifications of a rhyolitic lava. 
The investigations of the present writer upon a large col¬ 
lection of spherulitic rocks, showing many modifications not 
exhibited at Obsidian Cliff, have led him to the same con¬ 
clusion regarding his own material. He must not be under¬ 
stood to deny the existence of crystallitic aggregates, cumu- 
lites, or globospherulites, in glassy rocks, but he has seen no 
reason to believe that they are developed in the material he 
*The nature and origin of lithophysse and the lamination of acid lavas. 
Amer. Jour, of Science. 8°. New Haven, 1887 , Jan., vol. 33 , p. 46 . 
Obsidian Cliff, Yellowstone National Park. Seventh Ann. Rep. Di¬ 
rector U. S. Geological Survey. 8°. Washington, 1888 , pp. 249 - 296 . 
