1926 SYSTEMATIC 31 



Hounstout Cliff is 501 feet above mean sea-level, its base being 

 the perpendicular cliffs, which form the northerly rim of Chapmans 

 Pool — the clays with rotundum nodules. As the nodules are about 

 15 feet up, while the top of Hounstout consists of some 50 feet of the 

 lower part of Portland Stone Beds, some 70 feet may be allowed, leaving 

 430 feet for Portland Sands and so-called Kimmeridge Clay of 

 Hounstout-Chapmans Pool. Then, if the rotundum nodule-layer is 

 correlated with the Littleworth Lydite Clay, and that with the Lydite 

 Bed of Long Crendon, Bucks, while the basal Portland Stone of 

 Hounstout is correlated with late Behemothan, which is fairly obvious 

 from Mr. Cox's researches at Portland (Shell-Bed ; Proc. Dorset F.C., 

 xlvi, 1925, p. 113), there are some 430 feet of strata in the Isle of 

 Purbeck representing the same interval of time as only 15 feet of strata 

 do in the north-west quarry of Long Crendon, Bucks. In both cases 

 it may be surmised that the amount of strata actually preserved is less 

 than the amount of strata originally deposited ; but it is fairly obvious 

 that the actual amount deposited in the Isle of Purbeck was far greater 

 than that deposited in Bucks. But, again, it may be assumed that 

 in Bucks the amount removed was far greater in proportion to the 

 amount deposited, for in Bucks there are rapid lithic changes, signs 

 of redeposition and erosion ; while in the Isle of Purbeck the lithic 

 changes are fairly gradual. But suppositions of a far greater deposition 

 in the Isle of Purbeck, and of far greater denudation of less deposits 

 in Bucks, do not altogether explain the difference — ammonoids and 

 other fossils collected in Dorset suggest a whole series of episodes not 

 found in Bucks : whether the strata of such dates were never deposited 

 in Bucks, or whether they were deposited in an attenuated condition 

 and removed by penecontemporaneous erosion, may wait further 

 evidence. 



E. Hounstout and Chapmans Pool. (Blake's numbers cited 

 below, in brackets, refer to his section of St. Albans Head, about one 

 mile to southward — Q.J.G.S., xxxvi, 1880, 193. For the letters marking 

 beds, see Summary, sequence F — page 34). 



r. Portland Stone, Flinty Series [with Shell Bed at base ?] 

 (Blake's 10). 



Portland Sands — Cement Stone Series (Blake's 11, 12 and perhaps 

 13, for which he gives 85 feet) : — 



14. p. " Two blocks of sandy cement stones " show as two 

 prominent parallel bands beneath the stone series of Hounstout (Fig. i, 

 page 30) and in the west-facing cliff running towards St. Albans Head. 



13. n. A strong band of white argillaceous cement stone stands 

 out prominently in Hounstout, towards the lower part of the Cement- 

 Stone series. It yields many of the fallen blocks lying on the ledge 

 about the level of the old road (Fig. i, B), and it is the home of 

 specimens of Leucopetriies (PI. DCLXXVII). 



II. /. Below is a less prominent cement-band (Blake's 13 ?) of 

 a bluish colour, from which Mr. Waddington reports Thracia : it is 

 possibly the home of a biplicate Ammonoid (No. 4650, S.B. Coll.). 



Portland Sands — Sandy Series : — 



9. h. A little below 11 there stands out as a prominent band in 

 Hounstout Cliff a massive yellow and blue sandstone bed, some 5 feet 

 in thickness (Fig. i, h) : this is the most prominent datum-line in the 

 cliff, and blocks which have fallen on the ledge and shore below can 

 be easily identified : they yield biplicate ammonoids which have a great 



