328 The American Geologist May, i«.7 
hand, they can still dissolve small quantities of Na.^SO.+T Hg^? 
i. e., not yet saturated with respect to this substance,*' 
The two forms of calcium carbonate, calciteand arragonite, 
of the clay beds surrounded by the gently moving or quiet 
ground waters, corresjDond exactly to the two forms of sodium 
sulphate in the prepared solution. The more soluble arragonite 
continuously goes into the solution, thereb}^ over-saturating 
the solutioji with respect to calcite, upon the particles of which 
the calcium carbonate of the solution is deposited. Thus the 
calcite grows as the arragonite diminishes. If a clay bed 
were absolutely homogeneous all calcite particles would grow- 
in proportion to their surfaces, but in the beds as. we find 
them, some particles are much more favorably situated than 
others. 
Those particles of calcite which lie in those more permeable 
parts of the bed which form the water ways of the deposit, re- 
ceive materials in large quantities from the more rapidly 
moving waters and grow rapidly, while by far the larger part 
of the calcite, imbedded in the compact, almost impervious 
clay, can be reached only by small quantities of the very slow- 
ly percolating waters, and can grow but slowly. These wa- 
ters, however, gradually remove the arragonite from these 
compact parts of the clay and as the natural course -of these 
waters is towards and into the more permeable places which 
form the main water ways, the dissolved material is taken 
there and deposited upon the growing concretions. 
The calcite appears to have no power to remove the clay 
from its place and so crystallizes around the clay particles, 
thus producing the appearance of a "patch work or fine mo- 
saic" which Gratacap found upon microscopic examination.* 
In this case the concretion forms as a consequence of a 
transformation of arragonite to calcite, a physical change of 
state in the segregating substance. If the humus acids have 
as much effect as Julien thinks possible, concretions may also 
be formed through a change in the solvent. According to the 
article of Julien referred to. some modifications of that com- 
plex group of the products of organic decay known as the hu- 
mus acids have a strong solvent action upon calcium carbon- 
ate while other modifications render it nearly insoluble. When 
*Am. Nat., vol. xviii, p. 890. 
