40 Mr. Webster on the Purbeck and Portland Beds. 



inch to an inch and a half in length ; but during the short time I was on the 

 spot I was not fortunate enough to find perfect specimens. This bed, which 

 is of a thickness that varies from 1 foot to 2 feet or more, passes gradually into 

 that called the Purbeck marble, which contains several univalves as well as 

 bivalves ; among which, one resembles that of the Petworth marble, and is 

 considered by some as a Paludina, although in general a smaller species of 

 univalve is most abundant*. The cavities of these shells are usually filled up 

 by a compact limestone, and between them is a calcareous matter with much 

 green earth and red oxide of iron. This circumstance occasions some speci- 

 mens of the marble to have a variegated and pretty appearance; in which it 

 differs from the Petworth, which is usually of a grey aspect. 



On each side of these beds of marble, which seldom exceed 6 inches in 

 thickness, are frequently layers of fibrous carbonate of lime and also of fibrous 

 sulphate of lime ; and considerable masses of the latter have sometimes been 

 found. 



The Purbeck marble was In ancient times much employed in England for 

 making the slender shafts for the columns in Gothic churches, and in them 

 many fine specimens of this rock may be seen ; but the introduction of foreign 

 marbles has occasioned its use to be discontinued. Occasionally, this marble 

 exhibits no shells, and it is then of a greyish colour, veined with a darker grey. 



The rest of the Purbeck limestone beds appear to be derived from frag- 

 ments of shells, which in most instances are sufficiently evident ; but the stone 

 is sometimes almost compact. The greatest part of these fragments appear to 

 me to have belonged to small bivalves; but it is extremely difficult to procure 

 such specimens as can enable one to speak with any thing like certainty as to 

 the species. That the univalves in the marble, as well as in the other beds, re- 

 sembled the Helix vivipara, was observed by Woodward long ago, and I have 

 alluded to this in one of my letters to Sir H. Englefield: but according to my 

 own way of viewing the subject, it would be incorrect to call this a decidedly 

 fresh-water formation, since I think this term ought to be restricted to beds 

 supposed to have been formed in lakes : and I have no doubt but that the 

 Purbeck beds in general contain a mixture of fresh-water with marine shells. 

 I possess some specimens in which are beautiful calcedonic shells, both fresh- 

 water and marine ; and one bed in particular, called Cinder, consists almost 

 entirely of a mass of oysters. Fossil fish, nearly as beautiful as those of Monte 

 Bolca, are not unfrequently found in the Purbeck beds, several of which 

 are in the museum of the Geological Society. Fragments of the bones of the 

 turtle are also of frequent occurrence, and several entire skeletons have been 



* Sometimes, however, layers are found with univalves as large as those of the Petworth 

 marble. 



