220 . Dr. Daubeny on Arragonite. [Sept. 



the two freshwater ones, which Mr. W. designates as such j con- 

 sequently that there are only known at Headen Hill, or Alum 

 Bay, two distinct formations above the chalk ; first, the sand and 

 plastic clay, including the London clay ; and second, the fresh- 

 water formation, consisting of several beds varied in their 

 contents. 



There is some reason to believe that the Woolwich beds may 

 be cotemporaneous with this " upper marine formation ; " for 

 many of the shells contained in it are species of freshwater 

 genera. That of Plumsted is much more evidently a marine 

 formation, if we are to form our judgment from the shells it 

 contains ; but I cannot see any reason for supposing the " crag" 

 to be identical with either. All the fossils I recollect to have ever 

 seen from it are decidedly marine, and the formation bears 

 evident marks of identity with alluvium. 



I shall close these observations with two upon the shells 

 contained in a bed of clay, 1 1 feet in thickness, placed over Mr. 

 W.'s upper freshwater formation, and which, he says, are 

 unmixed with any other species, and of such a singular charac- 

 ter, that Mr. Parkinson could not refer it to any known genus. 

 First, I cannot say it is absolutely mixed with any other species 

 here ; yet there is within a foot and above it a bed of ironstone, 

 two inches thick, on both sides of which are immense quantities 

 of the same paludina as is found mixed with lymnaei in some 

 parts of Mr. W.'s upper marine. Secondly, in generic charac- 

 ters, the small bivalve shell in question very nearly resembles 

 corbula ; but, though the hinge cartilage is internal, and the two 

 valves unequal, as in that genus, yet there are some differences, 

 and there is strong evidence of its freshwater origin about the 

 umbones which are eroded. Its recent analogue is described 

 under the name myalabiata, and figured from the Rio de la 

 Plata in the Transactions of Linnaean Society, vol. x. to xxiv. 

 f. 1, 2, 3, p. 326. Yours, &c. 



G. B. Sowerbv. 



Article X. 



On Arragonite. By the Rev. Dr. Daubeny. 

 (To the Editor of the Annals of Philosophy.) 



SIR, Magdalen College, Oxford, July 7, 1821. 



Dr. Clarke, of Cambridge, inhispaper on Arragonite, pub- 

 lished in the last number of the Annals of Philosophy, remarks, 

 that " although Kirwan, 27 years ago, conjectured that this 

 mineral contained strontian, and Prof. Stromeyer, of Gottingen, 



