100 Dr. Morley Daviet^ — Origin of Septariau Stmctiore. 



confined to the earliest region affected and delayed until after further 

 deposition had taken place around? These were questions to which 

 I could not suggest an answer, but unfortunately I never had them 

 pressed upon rae until on an expedition with Mr. E. E. Lowe, B.iSc, 

 of Leicester, the question was raised between us, and in a few minutes 

 it flashed upon me that the real explanation must be, not contraction, 

 but expansion. I have, delayed publishing this opinion for over two 

 years in the hope of getting some confirmatory evidence, but the 

 chances of coming across such are so rare that it seems advisable to 

 call general attention to the subject so that many observers may 

 look out for them. 



It is obviously more probable that the deposit of calcium carbonate 

 in the interstices of a clay should result in expansion than in 

 contraction. We may think of a concretion as formed by the addition 

 of successive thin shells to a minute original nucleus. This nucleus 

 may be thought of as so minute that it can expand witliout internal 

 strain. When the first thin concentric shell of clay immediately around 

 the nucleus was impregnated with calcium carbonate, it would expand 

 and tend to separate fi'om the nucleus ; but owing to the continuity 

 of deposition it would be too firmly welded to the nucleus to do this. 

 It would therefore exert a tensile strain upon the latter, which would 

 lead to the formation of radial cracks as the sectors of the nucleus 

 were pulled away from their centre. As each successive shell was 

 impregnated and expanded it would exert a similar strain upon the 

 shells within it and the nucleus, so that a series of radiating cracks 

 growing wider inwards would result. It might occasionally happen 

 (owing perhaps to a temporary interruption to the continuous deposit 

 of calcium carbonate) that it was easier for a newly-deposited shell to 

 separate from the next one witliin than to split it radially ; in that 

 case a concentric crack would be formed which would widen later as 

 its enclosing shell was in turn pulled outwards by the expansion of 

 still later- formed shells. 



Confirmatory evidence of this explanation is not easy to get. 

 It might be thought that where large septaria occur, not too near 

 together, in a well-laminated shale, the latter might be seen to be 

 more compressed above and below the nodules than between them. 

 But, apart from the difficulties of observation resulting from possible 

 slipping, etc., we must bear in mind that the general compression of 

 the shale is of later date than the formation of the septaria,^ and 

 it would be hopeless to discriminate between extra compression due 

 to the passive resistance of the already-formed nodule and that due 

 to its expansion during growth. 



The only direction in which a crucial test of my explanation 

 seems possible is that of the careful study of the relation of radial 

 and circumferential cracks, especially in regard to width, and I would 

 suggest to all geologists whose work lies among beds in which 

 septaria abound, to keep a look out in this direction. 



' This was pointed out by De la Beche (loc. eifc., p. 597). Fossils which are 

 tound in a crushed state in the shale are uncrushed in the concretions. This 

 is the case, for instance, with the Chi'istian Malford ammonites, as Pratt long 

 ago noticed. 



