T. Crook — The ' Shrinkage' of Septaria. 515 



the process would probably result in a series of concentric cracks 

 infilled with calcite, rather than in radiating cracks. There is really 

 no evidence that the cracks develop during growth in this way; and 

 until some proof is forthcoming that crystallizing force acts in the 

 way suggested, it seems more reasonable to adopt the view that the 

 cracks arise from shrinkage subsequent to growth. This shrinkage 

 view seems to be well in accordance with the facts, and accounts for 

 the results in a more satisfactory manner. 



In explaining the formation of the cracks by shrinkage, it is 

 important to note that the rate of radial growth of a nodule probably 

 diminishes very rapidly as the nodule increases in size. If we assume, 

 for the sake of argument, that its shape is spherical, then the surface 

 area of the growing nodule increases as the square of the radius. It 

 follows, therefore, that the surface of deposition when the nodule is 

 10 cm. in diameter is 10,000 times greater than it was when the 

 diameter was 1 mm., and 100 times greater than it was when the 

 nodule had a diameter of 1 cm. 



Not only is there this tremendous rate of increase in the surface of 

 deposition during the growth of the nodules : there is also a decrease 

 in the dissolving surface ; for the particles of less stable carbonate 

 from which the solution maintains its supply are continually 

 diminishing in size as the nodules grow. Until the contrary is 

 proved, it is reasonable to infer from these facts that the rate of 

 radial growth diminishes enormously as the nodule increases in size. 

 If so, its substance during growth will gradually increase in com- 

 pactness from the centre outward ; and the last-formed layers will 

 probably be very much more compact than the first-formed layers. 



We should expect that the first-formed part of a nodule would 

 incorporate more argillaceous material than the portions formed later. 

 Mr. Todd states that this is not the case ; but his statement is not 

 convincing. At least we may assume that the argillaceous material 

 of the earlier-formed portion is much wetter than that of the later- 

 formed portion of the nodule. 



We may still conclude, therefore, that the internal mass of the 

 nodule consists of wet argillaceous carbonate, and that it is surrounded 

 by an outer shell of drier and more compact material. The nodule 

 in this state will, like the enclosing sediment, continue to suffer from 

 the compacting process. It will lose water, and shrink. The more 

 compact outer shell will suffer little tangential contraction. The 

 contraction of the wetter interior will be more considerable, and will 

 be accommodated by a shrinkage of the mass on the outer shell. 

 This will result in the development of radiating cracks, which may 

 ultimately become filled with calcite. 



I see nothing in the facts so far adduced that invalidates this 

 explanation. It is singular that so little has been done in the way 

 of investigating the facts bearing on this subject, and that so much 

 has been left to speculation. Until a fuller investigation has 

 strengthened the case against it, I for one shall continue to hold 

 the ' shrinkage ' view. 



