THE MELBOTTRN ROCK ETC. 229 



pierced are frequently filled up with coarser material which now 

 surrounds the nodules. 



Zone of Belemnitella plena. — In thin slices under the microscope 

 the marly chalk which always occurs at the base of this zone is 

 seen to contain a much larger proportion of the coarser shell- 

 fragments, such as may be seen in the Lower Chalk, and much less 

 of the finer material, the bulk of the latter having probably been 

 carried away by current-action. 



It may occasionally be noticed that pieces of unaltered white 

 chalk are included in this marl. These invariably exhibit the same 

 structure as the Lower Chalk into which the marl graduates. Where 

 these fragments are numerous (Chalkshire aud Royston) the marl 

 appears to be mottled white and grey ; this we have called " Mottled 

 Marl " to distiDguish it from the " Marbled Rock," which to the eye 

 it much resembles. 



Above the marl there is usually a hard white rock. Its micro- 

 scopical characters are remarkably constant. The spherical bodies 

 {Globif/erina-GeWs) before mentioned are exceedingly abundant and 

 form a large proportion of its mass, shell-fragments being com- 

 paratively rare. 



In the descriptions of sections (pp. 221-223) attention was drawn 

 to the fact that at Royston, Litlington, and Melbourn, aud in a cutting 

 of the G-.N.R. at Cadwell, near Hitchin, a hard rocky nodular bed is 

 seen in the place of the hard white smooth rock which is a feature of 

 this zone in many exposures. 



From an examination of this " Marbled Rock " by means of 

 smoothed hand-specimens, or thin slices seen under the microscope, it 

 appears to consist of more or less rounded fragments of chalk, from 

 the size of an egg to that of a pin's head, set in a greyish matrix 

 composed largely of shell-fragments. There is no difficulty in 

 recognizing under the microscope that the structure of these frag- 

 mentary pieces of chalk is exactly similar to that of the hard white 

 bed. We think it is probable therefore that this bed was once 

 continuous, but has been broken up ; and portions of it in the form 

 of lumps or nodules give evidence of its former existence. The 

 appearance of these fragments does not favour the supposition that 

 they have travelled far, but rather that while the chalk was only 

 partly consolidated, it had been subject to the washing of a gentle 

 current which carried away the finer particles and left only the 

 Terehratulce and other shells, and such portions of the. chalk as 

 were able to withstand its action. This idea receives some support 

 from the fact that at Melbourn, as already noted, the hard white 

 chalk is seen passing into a nodular band having precisely the same 

 characters as those under consideration. 



Prom the peculiar and marked characters of this "Marbled Rock" 

 there is but little doubt that this is the same as that met with in 

 the Richmond well-boring, and described by Prof. Judd in his paper 

 read before this Society on " The Nature and Relations of the Jurassic 

 Deposits which underlie London.*' Indeed, the characters displayed 

 in some fragments from Richmond which Prof. Judd was kind 



b2 



