RHYTHMIC BANDING OF MANGANESE DIOXIDE 611 
An analysis of the material to determine the percentage of 
MnO, was made by Mrs. W. A. Tarr. It was found that it was 
merely a stain which was readily removed from the tuff by acids. 
The amount of MnO, present was 0.82 per cent. 
At the point where the material was collected the rhyolite tuff 
is about 100 feet thick. It is overlain by a basalt and rests upon 
an andesitic conglomerate. As none of the specimens could be 
located on the face of the bluff it is not known whether the structure 
formed after the material had accumulated in the talus or while 
it was still in place. Likewise it cannot be determined from which 
part of the bed of tuff the material was derived. 
Description of the rhythmically banded structure.—The light- to 
dark-brown color of the rhythmically banded manganese dioxide 
in the light-gray tuff gives the rock a very striking appearance 
(Fig. 1). The amount of manganese dioxide in each structure is 
small (see analysis above), for the tuff is merely colored and is not 
replaced in any way. This staining process was greatly aided by 
the very porous character of the material. Practically all the 
banded areas are circular or elliptical, the former shape predomi- 
nating, though a few. are irregular in outline. In size they vary 
from 39 mm., as a maximum, down to mere brown specks. 
All the structures show either eccentric or concentric banding, 
this being true of those which are as small as 1 mm. in diameter. 
The banding is very delicate in most of them and is due to variations 
in the shade of the brown color. The bands are usually about a 
fraction of a millimeter in width, but in some of the large structures 
the bands are as much as 2 mm. in width, and occasionally the 
central zone has a diameter of 9 mm. and is uniformly tinted 
throughout (see Fig. 1). There are sometimes ten to fifteen bands 
in these larger structures. The rings are not absolutely uniform 
in their spacing or in their intensity, but they are so nearly so that 
slight variations in the composition of the solutions would account 
for the differences. The photographs do not show the bands clearly 
because of the small differences in light values between the zones. 
(See the drawing, Fig. 2.) The manganese dioxide affects the 
color of the finer pumiceous material, but as a rule does not change 
the color of the fragments of quartz, biotite, and other minerals 
