The Genesis of Clay Sfoiies. — Nichols. 329 
a clay bed is so situated tliat tlie percolating waters contain 
any considerable amount of humus acids, the solution of the 
lime will proceed with comparative rapidity. Humus-like 
substances may also be formed in the deposit itself by the 
slow decomposition of the organic luatter in and around the 
shells themselves. These may provide a strong solvent for car- 
bonate of lime localized where their effect is greatest. 
Humus acids are very unstable and are constantly undergo- 
ing changes in composition. Through such changes, the 
humus acid losing part of its solvent power, becomes super- 
saturated with calcium carbonate and read}' to deposit it upon 
any calcite it may come in contact with. How powerful an 
element this may be in the formation of clay stones depends 
upon the amount of humus that the groundwaters may carry, 
and upon how far into the deposit these substances may pen- 
etrate without decomposition. That their action has been at 
times great, the iron carbonate concretions of the coal meas- 
ures formed through their agency give evidence. This action 
of the humus acids is physically similar to the deposition of 
vein minerals from ascending solutions which as they cool 
and reach regions of diminished pressure are able to hold less 
in solution. It is not probable that any progressive change of 
temperature of the solvent waters takes place in a space so 
limited as that of a single clay deposit, while the change in 
pressure is insignilicant. 
These principles brief!}' applied here to the commonest and 
simplest of concretions, the clay stones, appear to be applica- 
ble to all concretions. Although concretions contain some- 
what less energy than the disseminated particles from which 
they are formed, the difference is altogether too slight to have 
any appreciable influence on their formation and it appears 
that, in practice, some energy must be applied to cause their 
growth. This energy comes from a change of state, either 
physical or chemical, by which either the segregating sub- 
stance or its solvent is transformed from an unstable condition 
to one more stable under the conditions to which it is subject. 
Field C'oluvibian Museum. 
