86 Feather stonhaugh^s Geological Report. 



merly expressed on this subject, bat the multiplication of spe- 

 cimens of late, many of them in good preservation, have 

 decided the question. 



The wealden group is too remarkable to pass over. It 

 consists of a series of strata of limestone, sands, and clay, 

 deposited from fresh water, enclosed as it were between forma- 

 tions of marine origin. The fossil shells found in it are anal- 

 ogous to the shells now living in fresh water, such as cyclas, 

 unio, paludina, and melania, with the exception of some that 

 can live in brackish waters. In the Purbeck beds, a stratum 

 of oyster shells occurs in the midst of a series of other strata, 

 some exclusively containing fresh-water shells, and others a 

 mixture of both fresh-water and marine. These beds are not 

 only extremely interesting on account of the very rare nature 

 of some of their organic remains, which form the greatest or- 

 naments of geological cabinets, but on account of the illustra- 

 tion they furnish of the changes of level to which strata have 

 sometimes been forced by geological movements, as well as of 

 the fact that each stratum has in its turn been the bottom of 

 the waters, whether marine or fresh. In the Isle of Portland^ 

 a small tongue of land which projects into the English chan- 

 nel, near Weymouth, a fine building- stone is quarried, which 

 is of undoubted marine origin, and of course was once the 

 bottom of the sea. We have the perfect evidence of this an- 

 cient floor of the ocean having been uplifted beyond its level, 

 in another bed, superincumbent to the marine one below, con« 

 taining the remains of an ancient forest of cycadeae. The 

 stratum in which this forest grew, extends through a consider- 

 able area in England, and has been recognised on the opposite 

 French coast. The trees, now silicified, are in many instances 

 buried in a black earth, in which they grew, which is about a 

 foot thick, and is called by the quarry-men " dirt-bed." Some 

 of the trunks of the trees are thirty feet long, and have a diam- 

 eter of three or four feet. Stumps, also, from one to three 

 feet long, separated by the usual distance at which forest trees 



