616 W. A. TARR 
concentration occurred. As the solutions became weaker the rings 
would become farther and farther apart unless the concentration 
of the oxidizing solution became stronger. The width of the zones 
between the rings would depend upon the relative rates of diffusion 
of each solution. That there was a slight change in the concentra- 
tion of the solution as long as there was a source of the manganese 
is shown by the gradually increasing intensity in color in the outer 
portions of the structures. The larger fragments of minerals in the 
tuff are uncolored because of their density. 
The eccentric character of the banding is probably the result of 
several factors. The rate of diffusion of the two solutions was 
probably different in different directions. The rate of diffusion in- 
ward from the surface of the oxidizing solution was probably most 
rapid, hence the oxidizing solutions met the manganese solutions 
nearer the nucleus on the side nearest the surface. This inter- 
pretation is in accord with the fact that the majority of the struc- 
tures show wider zones of rings on the side opposite to the surface, 
where the rate of diffusion of the manganese solutions is greater 
than that of the oxidizing solutions. Another factor may have 
been the varying porosity of the tuff, although this is not so likely 
as the foregoing, because it is not probable that the majority of the 
banded structures should have the same variation in porosity on 
the same side. When the blocks of tuff were lying in the proper 
position downward diffusion as influenced by gravity might aid in 
producing the eccentric rings. Unlike in laboratory experiments, 
where the rhythmic banding is produced in a gelatine of uniform 
composition and density, the rhythmic banding in nature would 
find varying factors on all sides, hence one should really expect the 
banded structtires to vary from perfect rings. The varying width 
of the rings themselves is due to the variable character of the solu- 
tions. It would seem that this suggestion as to the origin is the 
better of the two and, if correct, furnishes an interesting example 
of rhythmic banding in rocks. 
Summary.—Rhythmically banded structures of manganese 
dioxide are found in rhyolite tuff near Tucson, Arizona. These 
structures are eccentric, which is an unusual mode of occurrence. 
