Review of Recent Geological Literature. 103 
ion ; these were turned over to Messrs. Waclisnaitli and Spring- 
er for description. 
Dr. ^^'achsnulth says that "some of the specimens are up- 
per Devonian or Subcarboniferous forms." The presence of 
the Pisocrinus gemmiformis.) which is restricted to tlie Niag- 
ara, the i)resence of many cystids, and other Niagara fossils 
prove that these crinoids are of the Niagara limestone. 
This limestone can be traced from St. Paul, along Flat Rock 
to the place where the celebrated Waldron (Niagara shales) 
beds overlie this limestone ; farther up the stream on Little 
Flat Rock the writer was shown a deep hole in the bed of the 
stream where the Hudson river shales have been reached and 
found to underlie this limestone; this proves clearly that the 
crinoids are Niagara although they resemble later forms. One 
remarkable fact is that these crinoids do not run through the 
over-lying shales. 
Dr. Frank Howard, however,found in this limestone acystid 
about the size of a hen's egg, while Dr. Washburn, of Waldron, 
also found the same species of cystid in the shale on Conn's 
creek. 
Many facts are yet to be worked out, which may result in 
many discussions between the paleontologist and stratigraph- 
ical geologist. 
REVIEW OF RECENT GEOLOGICAL LITERATURE. 
Obsidian Cliff, Yellowstone National Park. By Joseph P. Iddings. 
Pages 249-205; plates ix-xviii. (Accompanying the seventh annual 
report, V. S. geological survey.) The widely famed Obsidian Cliff, 
extending half a mile and rising 150 to 200 feet above Obsidian creek, 
which flows at its base, is situated about eleven miles south of the 
Mammoth Hot Springs, at an elevation of 7,500 feet above the sea. 
Its upper half is a vertical face of black obsidian, or volcanic glass, 
which has resulted from the rapid cooling of a perfectly fused, igneous 
rock, and the lower portion is a talus slope of the same material. The 
clifT presents a partial section of a surface flow of obsidian which 
poured down an ancient slope of rhyolite from the plateau lying to the 
east. Following the obsidian back from the face of this cliff up the 
hummocky surface it becomes filled with gas cavities and passes into 
banded pumiceous rock and finally into light-gray pumice. 
A remarkable columnar structure, similar to that of basalt, is devel- 
oped in the southern part of this clill', where the obsidian flow had 
more than its average thickness. The shining black columnar prisms 
rise from the top of the talus slope to a height of 50 or (.M) feet and vary 
