UPPER SECONDARY ROCKS. 105 
pierodactyles^ or flying lizards, varying from the size 
of a snipe to that of an eagle. Above the oolitic 
formation there is in England a peculiar local de- 
posite, confined to the counties of Dorset, Hants, 
Sussex, and Kent, called Purbeck strata and Wealden^ 
consisting of beds of clay, common sandstone, cal- 
ciferous sandstone, conglomerated sandstone, lime- 
stone, and ironstone, forming an average thickness 
of 1000 feet. This formation is particularly remark- 
able, from the circumstance that the fossil remains 
are of fresh-water instead of marine origin. Over a 
surface of some sixty miles in length and twenty 
in breadth, the limestone, sandstone, clay, and con- 
glomerate contain almost exclusively the remains 
of fresh-water animals and terrestrial plants, while 
immediately below we find an abundance of the re- 
mains of marine animals. These two formations 
are separated by a calcareous bed about two feet 
thick, upon which is spread a stratum of dark clay 
from two to six inches in thickness, containing fos- 
sil specimens of cycadite^ analogous to living spe- 
cies of cycas, " Thus," says Bake well, " at the dis- 
tance of two feet we find an entire change from 
marine strata to strata once supporting terrestrial 
plants ; and should any doubt arise respecting the 
original place and position of these plants, there is, 
over the lower dirt-bed, a stratum of fresh- water 
limestone, and upon this a thicker dirt-bed, contain- 
ing not only the cycadiae, but stumps of trees from 
three to seven feet in height, in an erect position, 
with their roots extending beneath them. Stems 
of trees are found prostrate upon the same stratum ; 
some of them are from twenty to twenty-five feet 
in length, and from one to two feet in diameter. 
The following section of a cliff in Dorset exhibits 
very clearly proofs of the alternation from marine 
strata to dry land covered with a forest, and of a 
subsequent submergence of the dry land under a 
river or lake which deposited fresh-water lime- 
stone. 
