Br. Morley Davies — Origin of Septarian Structure. 99 



upper lialf of the area and then dies away very rapidly round 

 a practically circular aperture ; the lateral denticles are shown to he 

 the remnants of a cross-bar by the preservation of such a bar in a few 

 instances. 



This species, which occurs from time to time at Trimingham, is 

 placed under Homalostega on account of its general similarity to the 

 preceding one. 



EXPLANATION OP PLATE IV. 

 (All figures x 12 diams.) 



Fig. 1. Mucronella (?) Spenceri, nov. Zone of Offaster pilula, West Meon, 



Hants. 

 ,, 2. MiLcronella (?) Spenceri, nov. Zone of Act. qiiadratus, East 



Tytherley, Hants. 

 ,, .3. Mucronella (?) Batheri, milii. Type-specimen. Trimingham. 

 ,, 4. Homalostega cavernosa, nov. Trimingham. 

 ,, 5- 

 ,, 6. 

 ,, 7. Homalostega Tttlcani, nov. ,, 



II. — The Origin of Septa kian Stkuctore. 

 By A. MOELEY Davies, A.E.C.S., D.Sc, F.G.S. 

 rnHE explanation of Septarian structure, almost universally accepted 

 J_ in England, is stated as follows by Sir A. Geikie : " Septarian — 

 a structure often exhibited by concretions of limestone and clay- 

 ironstone which in consolidating have shrunk and cracked internally " 

 {Text-looh of Geology, 4th ed., p. 136). " In many cases the internal, 

 tirst-formed parts of a nodule have contracted more than the outer 

 and more compact crust, and have cracked into open polygonal 

 spaces " (ibid., p. 647). 



A. H. Green similarly wrote: "Occasionally a contraction of the 

 interior, after an outside solid crust had been forn;ied, produced the 

 cracks of the septaria, in which percolating water deposited a crystalline 

 lining" {Physical Geology, p. 280). 



J. Geikie expresses himself more cautiously, as indicated by the 

 two words here italicized : " The cracks are widest towards the centre 

 of a concretion, and die out towai'ds its circumference, as if the 

 interior had contracted after the outside had dried and become 

 consolidated" {Structural and Field Geology, p. 119). 



If we go back to De la Beche, we find him also expressing himself 

 with great caution: '". . . the ordinaiy manner in which such 

 nodules are broken in the interior, the cracks not extending to their 

 exterior surfaces, as if there had been a shrinking of parts from the 

 centre outwards, so that the resulting largest openings were central " 

 {Geological Observer, 2nd ed., pp. 597-8). 



Por years I had, in teaching, adopted this explanation of septarian 

 structure by contraction of the inteiior, but always with an uncom- 

 fortable feeling that I could not maintain it under cross-examination 

 hj an intelligent student. Why should the deposition of calcium or 

 iron carbonate between the particles of an argillaceous rock cause 

 contraction in bulk ? If it did, why should that contraction be 



