386 «J. P. Iddings—Nature and origin of Lithophyse 
same temperature. ‘To determine the interdependence of these 
important phenomena it is obviously first necessary to devise 
methods of operating.on steel adapted to the interval in ques- 
tion. In other words, our introductory problem is the anneal- 
ing of steel without oxidation and without carburation, at 
measured (high) temperatures, during stated times. It is in 
this direction that we propose to have our investigation proceed. 
Washington-Prague, September, 1886. 
Arr. 1V.—The nature and origin of Lithophyse and the lamin- 
ation of acid lavas ;* by JoSEPH P. IDDINGs. 
THE Yellowstone National Park with its thousands of square 
miles of rhyolitic lavas, presents a splendid field for the study 
of the various forms of structure and crystallization assumed 
by acid lavas in cooling. And it is the investigation of these 
‘phenomena, observed while prosecuting the work of the U.S. 
Geological Survey within this region in charge of Mr. Arnold 
Hague, that has furnished the writer with the data upon which 
the conclusions here stated are based. 
Among the many phases of crystallization so richly devel- 
oped in the rhyolitic obsidian forming Obsidian Cliff, the most 
characteristic is the spherulitic, producing spherulites from 
microscopic dimensions to several inchesin diameter. They are 
crystalline bodies with a radially fibrous structure, which is 
often accompanied by a banding in concentric layers of differ- 
ent color and density. In thin section under the microscope 
they are seen to be composed of sectors of fibers, which between 
crossed nicols do not extinguish the light parallel to the length 
of the fibers which would produce a distinct shadow-cross, but 
at different angles, making many rays of shadow, which in the 
smallest spherulites frequently approach the form of a cross. 
These spherulites have been traced through gradations of 
microstructure to groups of granophyre feldspars of extreme 
minuteness, which appear to be composed of intergrown quartz 
and acid feldspar, and enclose trichites and microlites which 
also occur in the spherulites, so that the mineral composition of 
the spherulites is most probably quartz, acid feldspar and trich- 
ites of magnetite with augitic microlites. Chemical. analyses 
of the spherulites and the obsidian in which they occur show 
that the two are identical, and that a spherulite is only a par- 
ticular form of crystallization of the once molten glass. The 
largest spherulites have an earthy texture and in thin section 
* Extract from article to appear in the Seventh Annual Report-of the Director 
of the U. 8S. Geological Survey; published by permission of the Director. 
