THE 'DERIVED' FOSSILS. 41 



THE DERIVED FOSSILS OF THE PORTLANDIAN. 



At Swindon the Portlandian rocks appear under two very 

 different aspects. Below, we find the true marine Portlandians, 

 t at the top we meet with ' dirt beds ' and freshwater limestone 

 interst ratified with the marine limestones. For this latter set of 

 beds I use the name Swindon Series 1 . 



A. The Swindon Series. 



Remanie materials from the destruction of the Swindon series 

 are not conspicuous either from the Upware or Potton Neocomian 

 deposits, and they have not hitherto been recorded ; but at Brick- 

 hill undoubted fragments of the Upper Swindon Limestone are of 

 frequent occurrence. Anyone who has worked in the great 

 1 The section exposed in the Swindon Quarry in 1875 was as follows : 



h. Greensand, Neocomian? 



g. Limestone bed with casts of Portlandian fossils. It varies rapidly in 



thickness. 

 /. Limestone with profusion of fossils, moulds and casts, of Portlandian 



species, Cythercea rugosa, Cardium dissimile, Cerithium Portlandi- 



cum, &c. 



e. Thin clayey bed, much contorted in the quarry. 

 E 



d. Bed of cherty rock and band of purer chert with freshwater fossils of 

 Purbeck species [?] Valvata? Varies much in thickness. This bed 

 is identical in appearance with some of the true Purbeck series. 

 L c. The dirt bed. 



Unconformity 



g x C 6. Great series of sand rock, and some sand; in places false bedded. 



Fossils few (Trigonia, Ostrea). 

 g 2 [_ a. Hard limestone, made up of a great extent of the casts of shells. 



The beds a and b are thoroughly Portlandian, but in beds c to g we find some 

 beds of Purbeck type. The chert bed and cherty limestone at d are just like those 

 of the true Purbeck series, and one would unhesitatingly so call them were they 

 not overlain by the limestone/ and g, which are again Portlandian. But the latter 

 beds are of greater importance in geological classification, being, in the language of 

 Mr Blake, the more normal type, whereas the freshwater beds are truly episodal. 

 The marine limestones / and g are not like the marine beds of the Purbeck series, 

 bvit are truly Portlandian. And what we have here is the record of the gradually 

 diminishing but oscillating stages of the latest Jurassic sea, when land and fresh- 

 water conditions resulting in formations of the Purbeck types were produced in 

 Portlandian times. To this latter set of beds, ranging from c to g inclusive, I 

 apply the term Sicindon series. If we could trace this series southwards we should 

 doubtless find it passing up into the true Purbecks, which were laid down when the 

 boundaries of the old Jurassic sea had become much more limited. 



