RHYTHMIC BANDING OF MANGANESE DIOXIDE IN 
RHYOLITE TUFF 
W. A. TARR 
University of Missouri 
The material upon which this study is based was collected by 
the writer on the south slope of Tumamoc Hill, the largest of a 
group of low hills of the same name about a mile west of Tucson, 
Arizona. The Desert Laboratory of the Carnegie Institution of 
Washington is located on the north slope of this hill. The geology 
of this region is presented in a paper™ by C. F. Tolman, Jr. 
As the writer had been unable to find any description of a similar 
occurrence of an eccentrically banded structure of manganese 
dioxide in rocks, it seemed to him that a description of this occur- 
rence would be of interest. The structure will be referred to as 
rhythmic banding. This term seems to be the best, as the struc- 
ture cannot be called an orbicular structure, which it resembles to 
a certain extent, because that term is applied to certain crystalline 
aggregates in igneous rocks. 
Mode of occurrence.—The specimens of manganese dioxide were 
found in the talus at the foot of a bed of rhyolite tuff which out- 
crops on the southern slope of Tumamoc Hill. The tuff, according 
to Professor F. N. Guild,’ consists of volcanic ash with numerous 
inclusions of pumice and darker, more basic, fragments. ‘The vol- 
canic ash consists of glass, fragments of quartz, feldspar, ferromag- 
nesian minerals, and kaolinized material. Professor Guild gives 
the following analysis: 
OHO Ea AVR Lie ie ee HONORE UN SHIA CANIN es UMGLE, ds Ok 73.59 
JOSH Oi aNI Ls Preesenatiras Silla Mish gina ems na HG) cana 13.95 
FAO Ae Ge sears iDUs DAR HigAIt oft Wy SIE al foun att ely ie Alte 
IMDS Oar Cee ae OURS Me aN Ran eat es ae en On75 
TE aN ate Oe i tI rE RU mies 
This analysis shows that the rock is very siliceous. 
t Publication No. 113, Carnegie Institution of Washington, pp. 67-82. 
2F.N. Guild, Publication No. 113, Carnegie Institution of Washington, p. 81, and 
American Journal of Science, XX (1905), 314. 
610 
