parti] or the suales-with-' beef.' 95 



the same general structure and origin, while their 

 peculiarities can be traced to the crystallographic cha- 

 racters of the minerals present. 



(E) The Calcareous Nodules. 



The chief problem presented by the occurrence of calcareous 

 nodules ('cement-stones') in the Shales-with-' Beef ' is the 

 determination of their age relatively to the sediments. The 

 significance of the evidence has been discussed elsewhere, 1 and it is 

 only necessary to collect the clues that these deposits furnish as 

 to their own age. 



Conformable disturbance of the strata is frequently observed. 

 This has been shown, on experimental grounds, to be consistent 

 with either primary or secondary age, and recently has been 

 reported over fossil remains. 2 As a criterion of relative age it is, 

 therefore, useless. 



The nodules have the flattened ellipsoidal shape suggestive of 

 secon lary origin. Septarian structure is common : a feature due 

 to the drying of a colloidal centre. 3 This structure is not con- 

 fined to nodules of secondary origin, and is no indicator of relative 

 age. 



While the nodules are sometimes fossiliferous, they are more 

 frequently non-fossiliferous, and are distributed quite regardless of 

 the location of fossils. Lignite is an occasional inclusion, and two 

 types are found : one of a woody character, and the other black, 

 resembling jet, but very brittle and showing septarian fractures. 

 Plant-remains within the nodules, however, are in the same state of 

 preservation as those found in the shales. It may, therefore, be 

 concluded that nodular segregation occurred when the principal 

 stages in the decay of vegetable matter were completed. It will 

 be recalled that Dr. Marie Stopes 4 proved the contemporaneous 

 age of coal-balls conversely by the more perfect preservation of 

 included plant-tissues. 



In considering evidence of the distribution of the nodules, the 

 enquiry must be extended "more widely than the particular levels 

 witli which this paper is moi'e intimately concerned. Completely 

 measured sections of the Dorset Lias are given by H. B. Wood- 

 ward,"' and on these the distribution suggested below is based. 



The characteristics of the vertical distribution of the limestones 

 and nodular lines are set out in the diagrams of fig. 9, p. 9G. 

 In the curve on the right the intervals between lines are plotted 

 against their position ab'Jve the base of the Lias. The same 



1 W. A. Richardson, Geol. Mag. 1921, p. 120. 



2 H. L. Hawkins, ibid. p. 192. 



3 W. A. Richardson, ' On the Origin of Septarian Structure ' Min. Mag. 

 vol. xviii (1919) pp. 327-38. 



4 M. C. Stopes & D. M. S. Watson, Phil. Trans. Roy. Soc. ser. B, vol. cc 

 (1909) p. 1C.7. 



5 'The Jurassic Rocks of Britain, vol. iii : the Lias of England & Wales ' 

 M.iii. Geol. Surv. L893, pp. 57 et seqq., 195 el set/i/. 



