PORTLAND BEDS. 337 



PORTLAND BEDS. 



The Portland Beds, deriving their name from the Isle of Portland, 

 are generally divided into : — 



Portland Stone, 

 Portland Sand. 



The name Portland Stone has been in use ever since the stone 

 has been quarried ; perhaps the earliest geological description is 

 that of Webster in i8i6.^ The name Portland Sand was introduced 

 by Fitton in 1827.^ The term Portlandian has been used by 

 D'Orbigny. 



The Portland Stone consists of white shelly and oolitic lime- 

 stones, with layers and nodules of chert,^ attaining a thickness of 

 60 feet and upwards at Portland, where the beds are largely 

 quarried. The beds beneath consist of brown and yellow sands, 

 clays and marls, with bands and nodules of calcareous sandstone, 

 sometimes full of green glauconitic grains, and they attain a thick- 

 ness of about 80 feet. Nodular concretions of calcareo-siliceous grit 

 occur in the sandy beds in Oxfordshire, which, as Conybeare 

 remarked, from their size and grotesque appearance, formerly 

 attracted the notice of every one who ascended Shotover in follow- 

 ing the old London road. The term Pot-lids is applied to these 

 concretionary masses of sandstone, when of a lenticular form. 



In the Isle of Purbeck the thickness of the Portland Sands has 

 been estimated at from 120 to 240 feet, but as there is a gradual 

 passage downwards into the Kimeridge Clay, there is no definite 

 line. 



Amongst the fossils, many of which occur in the shape of casts, 

 are Ammonites gigantcus (known in Portland as 'Conger Eels'), 

 A. hiplex, and A. holoniensis. Belemnites are exceedingly rare. 

 Brachiopoda are also rare, but Rhynchondla Portlandica and Discina 

 have been obtained from the Sands, and Waldhcimia from the top 

 beds on the Dorset coast. The more abundant Mollusca are 

 Cardium disstmile, Lucina Portlandica, Trigonia gibbosa, T. incurva, 

 Perna mytiloides, Ostrea expansa, O. solitana, Exogyra briintniiana, 

 Myoconcha Portlandica, Pectcn lamellosus, Lithodomus Portia ndicics, 

 Cyrena rugosa, Buccifitim naticoides, B. angulatum, Plciuvtotnaria 

 rugata, the ' screw ' or Cerithiutn Portlandiciim, Natica elegans, N. 

 incisa, etc. Isastrcca oblonga (the Tisbury 'star' or coral) generally 

 occurs in chert. Remains of the Reptiles Ceteosaunis, Goniopholis, 

 and Stcneosaurus ; of Fishes, Pycnodus, etc. ; and fragments of 

 coniferous wood are sometimes met with. 



^ Sir Henry C. Englefield's Isle of Wight. The term Portland Lime was used 

 by the Rev. John Michell in 1788. 



- Phil. Mag. (1827), i. 139 ; see also T. G. S. (2), iv. 103. 



■^ Chert is an impure kind of flint, of varied colour, often breaking with a flat or 

 splintery, and seldom with a conchoidal fracture. 



