248 Obsidian in the Yellowstone National Park. [April, 
less marked, both in the upper and lower part of the cliff, and 
farther out seems to be entirely lost, the glassy rocks grading 
into the gray sanidine trachytes and obsidian porphyries of the 
surrounding hills. 
Extending upward from the edge of the promontory in a mod- 
erately gentle slope are four or five hundred feet of obsidian 
strata that exhibit some most interesting characters. There is no 
heavy mass of pure glassy rock, but a succession of irregular lay- 
ers of a dozen or more varieties of spherulitic obsidian, obsidian 
porphyries and breccias. The colors of these rocks are exceed- 
ingly varied, the prevailing blacks giving way to reds, browns, 
greens and the richest possible marblings and mottlings. 
One of the most striking characteristics of these rocks are the 
spherulitic concretions which occur to a greater or less extent in 
all the varieties. These bodies seem to prevail in the ashy-like 
bands or layers which, in the more compact mass toward the base, 
are frequently contorted, giving the rock the appearance of a 
banded and contorted gneiss. The ashy- appearing layers are 
probably composed of the same material as the concretions, since 
when we split the rock with the bands, the surfaces of the gray 
bands next the glassy layers are simply a connected or coales- 
ced series of nodes or hemispheres which have the usual appear- 
ance of the more isolated concretions. Where the concretions 
are scattered throughout the glassy mass, they are globular or 
composed of a cluster of globes. They have, in most cases, a 
distinctly radiated structure, with not infrequently concentric lay- 
ers near the surface. The interior is gray or pinkish-gray, and 
the surfaces, pinkish or flesh colored. 
In the coarsely columnar part of the wall the spherulites are 
often a foot or more in diameter and appear much flattened and 
distorted. It is probable that these irregular forms are produced 
by the coalescence of a large number of smaller ones, as there 
are apparently many centers of radiation. Large beds of the 
rock seem to be made up almost wholly of the concretions, and 
where decomposed, a mass of coarsely cellular or honey-combed 
obsidian remains. The brecciated beds consist of an ashy 
matrix in which are imbedded angular fragments of every variety 
of the brilliantly-colored spherulitic and ordinary obsidians. 
The collection of hand specimens made at this place is very 
complete, numbering upwards of three hundred. Their examina- 
