320 Dr. PiTTON on t/ie Strata below the Chalk. 



feet', and in Buckinghamshire still less^. Mr. De la Beche estimates the whole formation at 

 250 feet^. The aggregate thickness, therefore, of the three groups which compose the Wealden, 

 cannot, probably, be less than 800 feet. 



Portland stone. — This formation being well defined and fully exposed, its thickness is well as- 

 certained; and in Portland Island it seems to range between 60 feet"* and about 70,* ; at Swindon 

 it is about 60 to 65": at Great Hazeley in Oxfordshire 27 feet' ; at Brill, about 23 feet" ; 

 near Quainton and Whitchurch in Buckinghamshire, from 4 to 20 feet". 



Portland sand. — Near St. Alban's Head, Isle of Purbeck, 120 to 140? feet'"; in the Isle of 

 Portland, 80 feet " ; near Thame in Oxfordshire, about 50 feet'*. 



Kimmeridge clay. — The thickness of this group is uncertain, and seems to vary much in dif- 

 ferent places. On the coast Messrs. Buckland and De la Beche assign to it 600 feet, at Kim- 

 meridge Bay '5; but they state the thickness at Ringstead Bay to be only 300 feet'*. At the 

 Headington quarries in Oxfordshire, I found, in one of the pits, only 20 feet of clay '*. The 

 general thickness is stated by Mr. De la Beche to be 500 feet'®. 



Oxford oolite. — On the coast near Weymouth, 150 feet". The whole, in Wiltshire, according 

 to Mr. Lonsdale, about 200 feet 's ; Mr. De la Beche, 150 feet '9. 



Oxford clay. — On the Dorsetshire coast, near Weymouth, 300 feet 2": at Lynn in West Nor- 

 folk Mr. Rose assigns to this formation the whole depth of a well, which would make the thickness 

 about 630 feet^' ; but part of this depth may possibly be Kimmeridge clay. Mr. De la Beche's 

 estimate for the whole formation is 600 feet**. 



(164.) Order of Geological Events. — The succession of events which the 

 preceding- pages demonstrate, has been already detailed in some other pub- 

 lications* ; but they are too important not to be recited here. The evidence 

 bearing- on the following- propositions, derived from the South Eastern Coast, 

 the Vale of Wardour, and Buckinghamshire, is all in perfect harmony; and 

 the inferences are so directly connected with the facts, as to be rather corol- 

 laries than deductions from them. 



1 . Deposition, beneath the sea, of the following groups of strata, beginning with the lowest : — 



a. Oxford oolite, b. Weymouth and Kimmeridge strata, c. The Portland sand and stone. 



2. Elevation of the Portland strata above the sea. 



3. Submersion, by fresh water, of the newly disclosed land. 



i. Deposition beneath fresh water, above the Portland strata,— first, of a thin bed of clay ; and 

 then of a crust of freshwater Umestone, the "skull-cap." (111.) p. 224, and (113.) p. 226. 



' Supra (132.), p. 251. ^ (^150.), p.285-6. ^ Tabular View. " Webster, Geol. Trans, 

 vol. ii. p. 42, 43 ; and PI. VI. fig. 3. * Buckland and De la Beche, PI. III. fig. 1. of this volume, 

 e Supra(141.),p. 267. ' (144.), p. 276-277. « (148.), C. p. 280-1. » (152.), p. 1^88, 290. 

 '« (106.), p. 212.— Section, fig. 5. Plate X.b. " Buckland and De la Beche, p.li). of this 



volume. '* Supra (148.) p. 282-3. '^ Page 19. of this volume — This estimate, however, 



appears to be too great : see above, (107.) p. 212-13. "> PI. III. of this volume, fig. 1.; andp.22, 

 1.6. '=> Supra (146.), p. 278-9. '« Tabular View. '7 Buckland and De la Beche, 



p. 23. of this volume, 1. 2. from bottom. '« Geol. Trans. 2nd Series, vol iii. p. 261. 



'" Tabular View. *o Buckland and De la Beche, p. 28. of this volume. *' Rose, p. 174. ; 



supra (161.) p. 316. *- Tabular View. 



* See the works of Mr. Mantell ; Dr. Buckland's and Mr. De la Beche's paper in the present 

 volume ; and the "Geology of Hastings", p. 79, &c. 



