2^22 



KEV. J. F. BLAKE ON THE 



meratic base, and are underlain by a local bed of clay, more allied to 

 the beds below. The Portland Sands are found below this in the 

 form of extremely fossiliferous sandstones or shell masses, which are 

 glauconitic. They contain a well-marked fauna almost restricted to 

 them. The base of the series is a thick mass of sands with huge con- 

 cretionary masses in the lower part. 



The districts of Oxfordshire and Buckinghamshire are made on 

 the same model. The " Purbeck " beds of the latter county lie 

 uniformly on the Portlands as in the south ; but the underlying lime- 

 stone corresponds to the Trigonia-'bed of Swindon. Hence the former 

 are of earlier date than the Purbecks of the latter place, and still 

 earlier than those of the Isle of Purbeck, and were formed m Port- 

 landian times. There are here still older deposits than at Swindon, 

 which may yet be reckoned as belonging to the Portland Stone, 

 consisting of redeposited beds with strange fossils. The fossiliferous 

 Portland Sand is much diminished in importance, but continues its 

 glauconitic character to the extreme north ; while the lowest sands 

 with their rounded masses are continued to Shotover and Thame, 

 and there almost disappear. 



Thus the Portland Sand had two maxima, one to the north of 

 Tisbury and one to the south ; but with regard to the Portland 

 Stone, the oldest beds are found in the north, and as we go south 

 later and later deposits are successively introduced before the traces 

 of freshwater conditions appear. These were the final result of the 

 gradual elevation in all cases, and were not of the same age through- 

 out, but followed immediately on the period of the beds below. 



It may be of interest to indicate the exact age of the different 

 beds of the Portland series which are used for building-stones. In 

 each case they owe their qualities to local conditions, and are not of 

 necessity of any value because they can be geologically called " Port- 

 land Stone." 



Building-stones of the Portland Series. 



Locality. 



Isle of Portland 

 Isle of Purbeck . 



Vale of Wardour 



Swindon. 



Oxfordshire 



Buckinghamshire 



Part of the series. 



(1). Eoach. Shelly. 



(2). Whit-bed. The celebrated stone. 



(3) Top of the Flinty series. Grood stone. 



(7) Middle of the Flinty Suboolitic freestone. 



* series. 

 (2) A bed corresponding to 



the Whit-bed. 

 (6) Lowest part of the 



Flinty series. 



(2) Same part of Flinty Very irregular hard stone. 



series as the last. 



(3) Trigonia-heds below the 



last. 

 (3) Same age as the last. A fair stone. 

 (2) Same age as the last. Ditto. 



Observations. 



A beautiful white stone, 



ringing under the hammer 



A soft freestone, very sandy. 



Poor. 



