J. E. Todd — On Septarian structure. 363 



cracked portion, as the old theory postulated. The compound ones, 

 on the contrary, are very instructive. They show apparently 

 successive additions made on one side after another and sometimes 

 one overlapping another, as in figs. 2 and 3. To explain them by 

 shrinking we must assume that small bodies of very conti'actile 

 material had been mixed with less contractile in curious lenticular 

 shapes by some inexplicable process. And if that be admitted, the 

 cracks should be widest in the centres of the respective contractile 

 members, whereas the facts are that the widest part of each crack is 

 toward the centre of the whole. And the centre of the concave or 

 inner side of each addition is evidently lifted away from the one 

 under it. 



By a further study of Figs. 2 and 3 it will appear veiy clearly 

 that the most obvious and easy explanation is that there is first 

 a collection of molecules of calcium carbonate in the pores of the silt 

 around some nucleus, but the conditions are such that the deposition 

 is not entirely at the surface, but largely in a shell reaching to 

 a considerable distance below the surface. After the concretion has 

 attained a certain size the centre will cease to receive additions to its 

 mass, while the outer shell will continue growing both radially and 

 tangentially, with force not only sufficient to crowd back surrounding 

 and superincumbent material, but to also rend asunder the interior. 



Should this seem incredible, let us remember the power of 

 molecular attraction manifest in the tensile strength of wire, or in 

 the supporting strength of rock, or of steel. In all cases it is thfe 

 attraction of molecule for molecule, the apparently opposite directions 

 in which it is manifested being due to different mechanical relations. 

 The power of freezing water and the capillary action of water in 

 wood which was used by the ancients for splitting rock are also 

 illustrations of similar forces. 



It may be noted further that the first cracking of the interior takes 

 place when the thickness of the growing shell is several times 

 the diameter of the ruptured interior, and that the cracks spread 

 gradually toward the surface which they may eventually reach, 

 When that is attained, water charged with various minerals will 

 enter and begin filling the crevice. When lined with crystals 

 they constitute a form of geode ; when perfectly filled with 

 crystalline matter, a true septarian. 



Professor N. S. Shaler appealed to similar action in the formation 

 of geodes of the common form, and also ascribed the filling of many 

 veins to the expanding effect of crystallizing minerals.^ 



Dr. Davies argues in his paper that we should not hope to find effects 

 of the expansion of the concretions in the surrounding shale because 

 the compression of the shale was subsequent to the growth of the 

 concretion. That may be true in some cases, but the writer has 

 frequently noted in the shaly clays of the Pierre formation in South 

 Dakota a shell of cone-in-cone structure sirrrounding large concretions 

 of the sort under discussion. He has considered this a direct effect 

 of the growth of the concretions. 



^ "Formation of Dikes and Veins": Bull. Geol. Soc. Am., vol. x, 

 pp. 253 ff. 



