part 1] OF THE SHALES-WITH-' BEEF.' 97 



Thus the deposit exhibits the characteristics of rhythmic precipita- 

 tion. There is a region of close deposition, followed by gradually 

 increasing separation of bands to the top of the Middle Lias, above 

 which the occurrence of nodular limestones is not recorded. 



Dr. Lang has drawn my attention to the palseontological evidence 

 of the existence of non-sequences l that are not insignificant, and 

 point either to extensive thinning or to contemporaneous erosion 

 of many feet of strata, which might include limestone-bands. It 

 is obviously a question of the first importance to decide whether 

 or no the limestone suite is intact. The available evidence is not 

 very complete ; but two reasons may be given for considering that 

 the full rhythm is present, and has not been affected by non- 

 sequences. 



In the first place, it is scarcely possible that beds of dense 

 limestone should be contemporaneously eroded, without originating 

 coarse beds containing limestone-pebbles situated within a reason- 

 ably short distance of the original site. But there is no record of 

 such pebbles. 



In the second place, it is found that the horizontal distribution 

 of the limestone-nodules corresponds to the vertical. In the lower 

 levels continuous tabular bands of limestone, often showing nodular 

 structure, or passing laterally into dense rows of nodules, are 

 common. Higher in the sequence tabular bands disappear, and 

 close nodular lines are characteristic ; and, as the vertical interval 

 between the beds increases, so also does the horizontal distance 

 separating the individual nodules of a bed. Therefore, the same 

 rhythmic sequence is obtained, whether the horizontal or the 

 vertical relations are studied. And, since there are no signs of 

 successive independent rhythms, it must be concluded that the 

 limestone suite is the product of one process of rhythmic deposition, 

 not only secondary, but apparently later in date than any 

 contemporaneous erosion that may have affected any portion of 

 the deposits. 



One further point must be mentioned. In places occur ' in- 

 durated bands ' of hard calcareous marl. These are not regarded 

 as part of the limestone suite, because all members of the latter 

 show clear lines of nodular origin and largely displace the marls; 

 whereas the indurated bands are shales impregnated with, or 

 cemented by carbonates, and may be regarded as a distinct, perhaps 

 final phase of the deposition of calcium carbonate at a stage (later 

 than the 'beef') when any solution remaining in the sediment 

 finally dried. 



(F) Sedimentary History of the Beds. 



I venture, in conchision, to suggest an outline of the sedimentary 

 liistory of these beds in stages based on the preceding study : — 



(I) Sedimentation, during which contemporaneous erosion may 



1 Prominently advocated for the Lias generally by Mr. S. S. Buckman, 

 Q. J. G. S. vol. lxxiii (1917-18) pp. 257-78 : see especially figs. A & B, pp. 265, 

 272. 



Q. J. G. S. No. 313. h 



