508 MESSES. WHITE AND TREACHER ON THE [Aug. I906, 



Among the features of zonal interest possessed by this little 

 section, one notes the shortness of the interval between the Chalk 

 yielding remains of Marsupites and that containing ff aster pillida. 

 Dr. A. W. Rowe has shown 1 that the width of this highest division 

 (barren of Marsujpites) of the Ma7*sujpites-Ba.n& varies from 15 to 20 

 feet in the coast-sections of Dorset and Sussex. Here it is under 

 3 feet. Part of this shrinkage is doubtless accounted for by the 

 attenuation which the Mar sixties-Zone, as a whole, undergoes in 

 Berkshire and North Wiltshire, but it is highly probable that 

 the general thinning was aggravated in the neighbourhood of 

 Winterbourne. 



It will be observed, also, that Actinocamax granulatus, though 

 associated with Marsupites in bed (3), appears just where that 

 crinoid is dying out, and does not become common until the base of 

 the Actinocamax-quadratus Zone is reached. The two forms occur 

 together in the Phosphatic Chalk of Taplow, about 30 miles to the 

 east-north-east, and in the normal Chalk of Surrey, Kent, and 

 Yorkshire. The relatively-late appearance of A. granulatus here, 

 however, is in accordance with the facts observed in other places in 

 the western part of the London Basin : for the three poor sections 

 of the higher part of the Marsupites-B&nd, and the single workable 

 exposure of normal quadratics- Chalk with which we are acquainted 

 in that area, all yield this species, though sparingly ; while in only 

 one of the vastly-greater number of Marsupites-jiel&ing sections 

 has this fossil been forthcoming. The zonal distribution of the 

 species in this part of the country seems to be much the same as in 

 the coastal parts of Sussex and Dorset. 



The top of the pit above described lies between 30 and 40 yards 

 distant from, and 8 to 10 feet lower than, the boundary of the 

 Beading Beds of the Borough-Hill outlier on the south-west. 

 Assuming that the observed dip (of 3°) in that direction is main- 

 tained as far as the boundary just mentioned, there is room for 

 about 15 feet of Chalk between the Eocene base and the top of the 

 pit. Adding this 15 feet to the sum of the measurements of beds (4) 

 & (5), we obtain 20 feet as the total thickness of the Actinocamacc- 

 quadratus Chalk at its outcrop at this point on the eastern side of 

 Borough Hill. 





(b) Exposure 300 yards east of Lower Farm. 



At a point about 420 yards east-north-east of the phosphate-pit (a), 

 and at a rather lower level, a rat-hole in a clover-field gave an in- 

 dication of chalk near the surface. On deepening and enlarging the 

 burrow, we encountered an obviously-phosphatic chalk in small 

 angular blocks, so closely set as to leave no doubt that they belonged 

 to the topmost layer of the rock in situ. Except that phosphatizeM 



1 'The Zones of the White Chalk of the English Coast' Proc. Geol. Assoc, 

 vol. xvi (1900) p. 333 & vol. xvii (1902) p. 26. 



