1884. | Opinions upon Clay Stones and Concretions. . 887 
Sedgewick considered concretions very interesting, inasmuch as 
they indicate that the determining causes are due to some débris, 
either animal or plant, or some small invisible grain which he 
remarks is in accord with what experience has taught us in the 
precipitation and crystallization of salts, which ordinarily are 
determined by the presence of foreign bodies in the midst of a 
saturated solution. The same action, as seen in the formation of 
calculi in the bladder, he discerns in nature, and regards the pres- 
ence of strange bodies as formative of concretionary centers, 
where the material separated from the different beds has gathered 
together. The principle of all this he suggests may be bound up 
in the assertion that like seeks like. | 
M. Turpin, speaking of siliceous concretions in the chalk, says 
if we admit that the nodules of flint owe their transformation to 
the decomposition of innumerable plants or animals, which live 
either in salt or fresh water, and upon these marine or lacustrine 
floors the débris or the entire bodies of these beings fall and pile 
themselves up upon each other, so as to form great beds, more or 
less thick,, made up of everything, if we recall that these animals 
are made up in great part, first, living organic matter, second, cal- 
careous material, third, silica, these two last having been absorbed 
and secreted molecularly and confusedly in the interstices of the 
first; if, in this pasty bed, gelatinous and very liquid, that we may 
call darégine, a bed where all is mixed, we admit, as appears 
proven, the separation, more or less complete, of siliceous parti- 
cles, and the conglomeration of the first amidst the second, as the 
globules of blood and those of milk, for sake of comparison, 
Separate from the serum to form clots, we ean imagine that any 
Natural object, organic particle, &c., might form the center of 
their concretionary growth or be enveloped in their outward ex- 
tension. M. Virlet thinks M. Turpin proves too much, and that 
if such an attraction were universally so, clay beds or siliceous 
limestones would not be homogeneous but form themselves into 
beds of nodules of silica and lime. 
Lyell, in his Elements of Geology, thinks that the molecules 
were held in suspension in water; once deposited, those which 
have a similar nature appear to exercise mutual attraction upon 
each other and gather in certain spots, where they form heaps, 
nodules and concretions. He instances the celebrated beds of 
magnesian limestone in the north of England, where the size of 
