SPHERULITIC CRYSTALLIZATION. 
BY 
Joseph Paxson Iddings. 
[Read before the Society, April 25, 1891.J 
Having had occasion recently to study a new series of 
thin sections of the lithoidite of Obsidian Cliff, Yellowstone 
National Park, which were prepared for the educational 
series of rocks to be distributed by the U. S. Geological 
Survey, the writer has been led to review his previous work 
on the spherulites of the glassy and lithoidal rhyolite of this 
locality. The new thin sections present 26 examples of one 
phase of the rock, whereas the previous 50 or 60 thin sections 
represented its great variability, with more or less duplication. 
In the new thin sections the microscopic spherulites attain 
a degree of crystallization that permits their structure in 
many cases to be clearly observed. The use of the quartz 
plate^ giving the purple color (teinte sensible) between crossed 
nicols, for determining the relative values of the axes of 
elasticity throws additional light upon the character of these 
radially fibrous intergrowths. 
The results of the recent study corroborate the conclusions 
already reached, and extend our knowledge of the subject 
by making it more definite. The investigation also adds to 
the list of primary minerals in this rock three not previously 
observed, one of which is rarely, if ever, found as a constit¬ 
uent of volcanic rocks, and its presence in this one is a further 
indication of the part played by mineralizing agents in the 
original crystallization of the rock. The minerals are tour¬ 
maline and mica, with an occasional zircon. 
57—Bull. Phil. Soc., Wash., Vol. 11. 
( 445 ) 
