450 
IDDINGS. 
whenever the undertaking promises to furnish additional 
evidence of their mineralogical and crystallographic char¬ 
acter. Such have been the grounds for recording the follow¬ 
ing observations : 
The lithoidite from which the new thin sections were pre¬ 
pared is the same as that described in the papers on Obsidian 
Cliff already referred to. It forms the thinly laminated 
lithoidal portion of the obsidian flow a short distance north 
of the columnar glassy portion immediately on the road near 
the bridge. It is light, purplish gray, delicately banded 
by layers of different degrees of crystallization, the most 
highly crystalline being white, and the more or less glassy 
layers the darkest colored. It is filled with lithophysae of 
great beauty, which are very open and cavernous, the volume 
of the cavities frequently exceeding that of the solid portion 
of the lithophysae proper. 
In thin sections the rock is irregularly banded or mottled 
according as the sections have been ground across or parallel 
to the layers of lamination. The most crystalline parts are 
colorless, the dark gray portions are mottled with minute 
spots, and with a low magnifying power these dark portions 
are seen to be minutely spherulitic, exhibiting characteristic 
black crosses. The transparent parts are quite crystalline 
aggregations of tridymite or quartz and feldspar, often with 
numerous irregular cavities between the crystals. Occasion¬ 
ally these places contain irregular grains of fayalite, and still 
more rarely brown mica, besides scattered spherulites, which 
also border these more crystalline portions. 
The thin sections show a few of the larger spherulites 
which are porous, and in some places branching, arbo¬ 
rescent or feather-like growths of feldspar—in fact, all of the 
modifications of spherulitic crystallization described in the 
paper on Obsidian Cliff. 
Studied with higher magnifying power, it is seen that the 
finely spherulitic portions are crowded with trichites, which 
are more perfect as the spherulitic structure is more minute, 
but which lose their form and uniformly fluidal arrange- 
