124 C. P. Chaticin 8^ T. H. Withers— Chalk Section at Marlow. 



of Puzosia described/ wliicli were found in the weathered rock 

 immediately beneath the soil. About 10 feet above the level of the 

 Chalk Rock there is a prominent band of tabular flint which shows 

 step-faulting, the total downthrow being 2 feet to the north-east. 

 There are no well-marked lithological features above the tabular flint 

 band, so that either this band or the Chalk Rock can be used as 

 a datum-line. The interporiferous areas of the examples of Micraster 

 prceciirsor collected below the tabular band vary from sutured to feebly 

 inflated. Those from this point up to 4 feet above the tabular band, 

 have feebly to moderately inflated ' areas ' and belong to the upper 

 Holaster planus-zone type. Above this horizon the ' areas ' of the 

 Micrasters gradually become more strongly inflated, and not until the 

 height of 16 feet above the tabular band do they exhibit the features 

 of the Micraster cor-testudinarium-zone type. At this horizon occurs 

 a band of nodular flints, and this may be regarded as marking the 

 junction between the two zones. The thickness of the Holaster planus 

 zone, if we estimate the thickness of the Chalk Rock at 6 feet, is 

 therefore 32 feet. 



The following is the section seen, with the fossils collected from 

 the various beds : — 



Micraster cor- 



testndinarium 



zone. 



( Firm white chalk with many hands of nodular and tabular 

 / flhif, . 



Feet. 



23 



Solaster planus / 

 zone. \ 



flint 

 Micraster prtBcursor, Cardiasttr cotteauanHs. 

 /Greyish, lumpy chalk with tabular flint lines and small 

 scattered flints. One conspicuous band occurs about 



10 feet from the floor 26 



Micraster prcecursor, M. cnr-testudinariuin, Echinocorys 

 scutatus, var. gibhiis, Holaster placenta, Cidaris scrri- 

 fera, Dimyodon nilssoni, Inoceramiis sp., Ostrea sp., 

 Spondyliis latus, S. spinosus, Crania egnabergensis, 

 Rhynchonella reedcnsis, Terebratula carnea, T. semi- 

 globosa, Serpula ampullacea, S. Jluctuata, S. plana, 

 Terebella leiuesiensis, Coscinopora sp. 



Floor of pit. 



Chalk Rock. Massive cream-coloured limestone, with 



layers of green-coated nodules . . seen for 3 



Micraster leskei, M. prcecursor, Inoceramiis sp.. Area 

 (Barbatia) sp., cf. geinitzi, ? Solariella gemmata, 

 Trochus sc/iluteri, Turbo geinitzi. Nautilus sp., 

 Scaphites geinitzi, Pachydiscus peramplus, Puzosia 

 curvatisulcata, sp. nov., Terebratula seiniglobosa, 

 ? Parasmilia granulata, P. aff. serpentina. 



In a previous paper ^ we commented on the connection between the 

 thickness of the zone of Holaster planus and the development of 

 the rock bed at its upper limit, and stated that where this rock bed 

 was developed the thickness of the zone was considerably decreased. 

 It is interesting to note that the present section confirms our views 

 on this point. A comparison between three sections in the zone of" 

 Holaster planus in this part of the country will make our point clear. 



1 See Geol. Mag., February, 1909, jjp. 66-9, PI. II. 



"^ " The Zones of the Chalk in the Thames Valley between Goring and Shiplake " : 

 Proc. Geol. Assoc, 1908, vol. xx, pp. 412-13. 



