CONCRETIONS. 713 



Again, since, as I have described above, every layer is, as a rule, 

 capped by a thin lamina much finer and more impervious than the rest, 

 while the seams between the larainaj are often quite coarse sand, at least at 

 base, the water would move most freely in the lower and coarser portion 

 of the layer, and least freely in the finer clay laminaj above and below. 

 This would tend to confine the water within the limits of single layers. 

 Its motion would vary, also, by infinitesimal gradations with every varia- 

 tion in the permeability of the layer through which it was passing and 

 with every diff'ereuce in the force by which it was moved. 



If we suppose, now, that from any cause a precipitation of the calcic 

 carbonate is commenced at a given point or in a limited area within the 

 coarser layer — a precipitation, that is, taking place in the interstices between 

 and among the grains, cementing- them together without greatly moving 

 them from their places — the waters deprived of the carbonate held in solu- 

 tion would move away and be in turn replaced by other water, still con- 

 taining in minute quantity the same salt; or, by difi"usion, a new portion of 

 .the salt would replace what had been precipitated, and, the same cause con- 

 tinuing to act, there would thus follow a continuous slow crystallization 

 within the limits and during the continuation of its efi'ective action. Nor 

 would the process of necessity cease with the exhaustion of the supposed 

 cause by which it was initiated. As a crystal already started grows in an 

 undersaturated solution, the calcite already crystallized out among the 

 grains of the clay would form a group of centers around which more and 

 more new portions of the same would crystallize until a compact mass had 

 been formed, but little permeable to water, which would then continue to 

 inci'ease by superficial accretion, en-^-eloping in its growth the fine sand, as 

 a fog- spreads among the trees of a forest. 



If the above considerations be founded on fact, it follows that w^e have 

 only thus far to seek for a cause which may start the growth of a concre- 

 tion around a given spot in the clay, its continued increase being adequately 

 explained by a reference to the laws of difi'usion and crystallization. So 

 far as all the claystones of the valley are concerned, this initiating- cause 

 entirely eludes our observation. Very rarely a pebble or a few grains of 

 sand are inclosed wholly or partly by the concretion, but I have never 

 seen one occupying the exact center, and they do not seem to have been 

 the exciting cause of its growth, but rather seem to show that a reg-ular 



