Geohxjicnl Causes of the Scenery of Yelloti'stone Park. 159 
colorless glass set with augite prisms, the augite being pre- 
cisely similar to that of the enclosing andesite and reaching 
the size of the ordinary phenocrysts of that mineral. 
Freshness of the material. For the most part the speci- 
mens are of perfectly fresh rock. As they consist of small 
pieces picked up on the mountain this fact has significance 
as indicating the comparative recenc}' of the outflow. But 
three specimens show any sign of decay. Two of the pieces 
of pumice have their glass reddened by oxidation of the fer- 
rous oxide, and the pitchstone has suffered slightly from the 
same cause. Specimen 8 likely represents an older flow, as its 
pyroxene has suffered some decomposition. The smaller crj'S- 
tals of that mineral have been wholly, and the larger external- 
ly, converted into the reddish-brown mineral which is so fre- 
quentl}"^ produced by the decomposition of pyroxene in such 
rocks. 
Conclusion. The recent lava of Bogoslotf Id., Alaska has 
been shown by Mr. J. S. Diller to be hornblende-andesite.* 
With this exception no andesite has been reported from Alas- 
ka as far as the writer is aware. Its occurrence here is there- 
fore of interest, especially in connection with the occurrence 
of similar lava throughout the whole extent of the American 
Sierra belt. 
SOME GEOLOGICAL CAUSES OF THE SCENERY 
OF YELLOWSTONE NATIONAL PARK. 
By A. R. Crook, Evanston, 111. 
For most of our knowledge of the underlying causes of the 
scenery of the Yellowstone National park we are indebted to 
Holmes, Hague, Iddings, Weed, and others of the United 
States geological survey. The results of their work should 
be in the hands of every traveller to the region, since the pleas- 
ure of the trip is greatly enhanced by some knowledge of the 
cause of the phenomena and by some idea of the forces which 
have produced the mountains, hills, and valleys, the heat of 
the region, the geysers, and the wonderful coloring. A student 
of creation here finds splendid illustration of the dynamics of 
mountain building, of the formation and metamorphosis of 
^Science, Jan. 1885, p. 66. 
