86 Featherstonhavgh s Geological Report. 



rneily expressed on this subject, but 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 cyeadeae. 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 w 7 hich they grew, which is about a 

 foot thick, and is called by the quarry -men u dirt-bed.&quot; 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 



