RHYTHMIC BANDING OF MANGANESE DIOXIDE 613 
8 mm. from one side and 20 mm. from the other, yet the structure 
was spherical. The majority have their nuclei to one side of the 
true center. It was this peculiar zoning that first attracted the 
writer’s attention. 
The only regularity of arrangement of the banded structures in 
the tuff is their relationship to the surface, as they are more numer- 
ous near the exposed surface of the frag- 
ments of tuff. All of the specimens on hand 
show weathered faces on one or more sides, 
thus indicating that the structures are defi- 
nitely related to the surface of the talus blocks 
from which they were obtained. The largest 
structures are usually an inch or so from the 
surface, the intervening spaces being filled Se ores 
with numerous smaller banded areas, which eccentric structure. Ac- 
not infrequently coalesce. A larger one may _ tual size. 
partially or wholly envelop a smaller one, 
’ apparently without affecting its color in any way. Again, two 
large ones may be so close that only a line separates them, yet each 
is distinct, or they may interfere and produce a lobate structure. 
A rather striking feature is the position of the nuclei in those 
forms that are eccentric and that lie near the weathered surface of 
the talus block. The nuclei are on the side of the structure which 
is nearest to the weathered surface, and the bands widen upon the 
opposite side. This may be definitely connected with the origin of 
the rhythmic banding. The largest structures and also those most 
nearly circular lie farthest below the surface. These, as a rule, are 
also the most perfectly banded structures. 
Suggestions as to the origin of the rhythmic banding in the man- 
ganese dioxide.—The origin of the rhythmic banding is an interest- 
ing problem, and the two following suggestions appear to be the 
most feasible explanations of the phenomena: (1) the manganese 
was leached out of the tuff and deposited near the surface of the 
blocks, or (2) some mineral in the tuff furnished the manganese, 
the dioxide forming in the zone around it. 
1. That the aggregation of the manganese into rhythmic zones 
might be due to the same process that produces the well-known 
