'The Geiiefyi.s of iUcnj Stoue.s. — JVichols. 325 
feldspars for instance, and in this fine state of division it is 
well known that siieli minerals decompose rapidly if other 
conditions are favorable.* 
To recapitulate, a recently formed clay bed may have as a 
source of lime for concretions : 1, comminuted lime and calcite ; 
2, shells in situ, and in the ease of glacial clays; 3, comminu- 
ted land and water shells, and 4, mineral flour containing lime 
ready to be liberated as carbonate under favorable conditions. 
This carbonate of lime is distributed through beds of clay. 
Such clay beds are never homogeneous. There are incipient 
partings parallel to the bedding and thin layers of coarser 
material as well as contraction joints and faults, all of which 
atford a comparatively easy passage to water, while the body 
of the clay is almost impervious, allowing onl}^ an extremely 
slow seepage. In addition there may exist planes of tension 
where the cohesion of the clay i.s weakened, and along these 
the waters may circulate with comparative freedom. 
During the progress of segregation of the concretions, the 
beds are saturated witli ground waters. These groundwat- 
ers charged with carbonic acid and perhaps with other solvent 
agents, take up and carry in solution portions of the carbon- 
ate of lime they meet, and it is through the action of these 
waters that the concretions are formed. 
This much is generally understood, butexcept for an impor- 
tant paper by Julien upon the humus acids, f to be referred to 
later, but little has been written upon the modus operditdi of 
the formation of concretions, and what has been published 
refers to the chemical changes by which the material of the 
concretion is formed rather than to the formation of the oi)- 
ject itself. 
Dana has suggested in the last edition of liis Manual;|; that 
clay stones are formed by deposition of the lime contained in 
waters by their evaporation during a drying season. It is ev- 
ident that the waters circulating in a bed of clay ma}' under- 
go constant changes in volume, rapidit}'^ of movement, degree 
of concentration and perhaps even in temperature and amount 
of contained carbonic acid or other solvent. If waters sub- 
*Roger8, Am. J. Sci., Series II, vol. 5, pp. 401-5. 
tJulien: On the Geological Action of the Humus Acids, Proc. Am. 
Assoc. XXVIII, 1879, p. 311. 
JDana: Manual of Geology, 4th ed., p. 139. 
