CONCRETIONS. 715 



them. Wlieii we consider, however, the extreme facihty with wliich cak'-ic 

 carbonate is brought into sohition and again precipitated, it woukl seem tliat 

 even a shghter cause might start the growth of a concretion; the decompo- 

 sition of a grain of feldspar setting free an alkah which woukl appropriate 

 the carbonic dioxid, or even the slight difference of specific heat of dif- 

 ferent minerals starting feeble thermo-electric currents between difterent 

 grains, causing them to become points around which crystallization would 

 commence, somewhat as a crystalline precipitate forms along the scratches 

 of a beaker when it is rubbed with a glass rod. 



By cutting down a smooth surface at right angles to the lamina? in a 

 mass of the clay one can sometimes find early stages in the formation of 

 concretions which illustrate what has been said. There are small spaces 

 confined between two layers of the finer clay, rudely spherical in outline, 

 though not sharply defined, which cut with somewhat greater difficulty 

 than the cla)' aroiind, are lighter in color, and eftervesce abundantly with 

 acid, while the clay around shows but slight signs of eff'ervescence. In this 

 case the continued accumulation of carbonate within the limits already 

 marked out would produce the small sphere which may be looked upon as 

 the normal form. The subsequent growth would be by additions to the 

 outside of this, determined by the varying permeability of the clay, pro- 

 ducing forms fluted on the edges where the concretion spread through sev- 

 eral layers — the constrictions answering to the denser layers, the convex 

 projections to the coarser — and flat disks where the increase was confined 

 to a single lamina. Finally, the coalescence in various ways and various 

 degrees of two or more separate spheres or disks would form all the variety 

 of compound and imitative forms. 



In another case I discerned, on cutting across the clay, that ferric 

 hydroxid had been precipitated in a regular hollow shell i'"™ thick, flat 

 spheroidal in shape, 14°"" in greatest diameter; and within and concentric 

 with the first shell was a second, similar in shape and thickness, 8°"° in 

 diameter. Here the influence of an atmosphere of decomposition surround- 

 ing a central body, which precipitated the iron salt as the latter came from 

 all sides within the sphere, seems to be more apparent. Finallv, the annu- 

 lar claystones — perfect rings of various sizes up to 10 inches — may be 

 ex})lained in the same way by supposing the emanation from a decomposing 

 center to be confined within a single layer, and thus to assume a discoid 



