614 W. A. TARR 
dark coating called “desert varnish,” which is found on surface 
materials throughout the desert region appears to be plausible. 
This coating is thought to be due to the evaporation of water drawn 
from the interior by capillarity, the salts in solution being deposited 
at the surface. The writer has never been able to find an exact 
statement of the chemistry of this process, and he feels inclined to 
doubt the applicability of the method suggested, because iron and 
manganese salts are very difficult of solution even under favorable 
conditions, and it is well known that the arid climate of Arizona 
and New Mexico favors oxidation and tends to inhibit reduction, 
which is a necessary step in the solution of the salts of these ele- 
ments. 
However, in the case of the tuff the solution could circulate 
through it readily, as it is very porous, and if proper solvents were 
present some manganese might be dissolved. Upon evaporation 
at or near the surface, where oxidizing conditions prevail, the man- 
ganese would be thrown down as the oxide. Once started, the 
structure would grow by fresh additions from the outside. As the ~ 
solutions were moving outward it is to be expected that the growth 
on the inside would be most rapid, and thus the eccentric rings 
would develop. Variations in the amount of manganese and the 
hydration of the resulting oxide would account for the color varia- 
tions. At a distance from the surface the structure would be 
nearly circular, because of the more uniform addition of material. 
It should be noted, however, that rhyolites are usually very low 
in manganese, which is against the theory that the manganese has 
been derived from the rhyolite tuff. Such rocks nevertheless 
occasionally contain minerals which are high in manganese, the 
garnet spessartite being especially.common in them. Such a 
mineral, even though present in small amounts, when broken and 
scattered through a block of tuff might furnish a fair source of 
manganese. Likewise biotite sometimes contains manganese, and 
considerable biotite (nearly always hydro-biotite) occurs in the tuff. 
These are therefore possible sources of the manganese. 
2. The manganese dioxide of the rhythmically banded struc- 
tures may have originated from and developed around a fragment 
