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1884.] Opinions upon Clay Stones and Concretions. 891 
strong, did not entirely remove the calcite, it lingered still, en- 
trapped in the clay particles. 
The assumption which may combine these features and accords 
with microscopic examination seems to be this: That in the 
clay beds where these dogs occur there were variously shaped 
spots or irregular separations between layers where moisture 
remained for a long time, keeping the ‘clay in a plastic, more or 
less liquid form, and that with the withdrawal of water to these 
points, the soluble carbonate of lime was also gathered there. 
This latter became concentrated by the contraction of the magma, 
or through introduction of more carbonate of lime in solution in 
percolating waters. i 
Upon concentration the carbonate of lime crystallizes within 
the mass and one crystal speck thus appearing would form a 
center of growth around which the new crystals would gather, 
drawing in clay with them. If this action occurred in a narrow 
seam-like plane the concretions would be flat, if in a wider area 
more spherical Sphericity of growth seems an ultimate princi- 
ple, but contraction of space and thinness of the concreting layer 
for the most part causes the concretions to expand symmetrically 
all around. : 
Where spots of this character coincide one over the other in 
rising layers of the clay beds, a pillar is formed, where they con- 
nect obliquely a slanting pile of disks is made (Pl. xxvii, Fig. 6). 
As the material in one of these areas becomes consolidated, we 
may suppose the upper surface to become denser and crystalliza- 
tion beginning at a number of points the surface is broken up in 
a number of separate forms. Thus the small wart-like bodies 
would seem to have formed quickly and to have become fixed 
at once. 
This growth may have been quite gradual or quite rapid, but 
in all cases due to the concentration of the calcite solution in the 
clay magma and then its crystallization. It could have been in- 
terrupted, irregular, more on one side than another, two or three 
or more concretions may have originated at the same time and 
grown towards each other, or interfered and enveloped each other 
in endless diversity of ways. One concretion may have become 
the center around which others grew, or a small concretion may 
have formed in the midst of a larger one, the quantity and state — 
_Of the carbonate of lime varying from point to point. A certain 
