344 
ELEMENTS OE GEOLOGY. 
Fig. 331); and many generations of these molluseoids had 
succeeded each other in the pure water before they became 
fossil. 
We may, therefore, perceive distinctly that, as the pines 
and cycad eons plants of the ancient “ dirt-bed,” or fossil 
forest, of the Lower Purbeck were killed by submergence 
under fresh water, and soon buried beneath muddy sediment, 
so an invasion of argillaceous matter put a sudden stop to 
the growth of the Bradford Encrinites, and led to their pres¬ 
ervation in marine strata. 
Such differences in the fossils as distinguish the calcareous 
and argillaceous deposits from each other, would be de¬ 
scribed by naturalists as arising out of a difference in the 
stations of species; but besides these, there are variations in 
the fossils of the higher, middle, and lower part of the oolitic 
series, which must be ascribed to that great law of change 
in organic life by which distinct assemblages of species have 
been adapted, at successive geological periods, to the vary¬ 
ing conditions of the habitable surface. In a single district 
it is difficult to decide how far the limitation of species to 
certain minor formations has been due to the local influence 
of stations^ or how far it has been caused by time or the law 
of variation above alluded to. But we recognize the reality 
of the last-mentioned influence, when we contrast the whole 
oolitic series of England with that of parts of the Jura, Alps, 
and other distant regions, where, although there is scarcely 
any lithological resemblance, yet some of the same fossils re¬ 
main peculiar in each country to the Upper, Middle, and 
Lower Oolite formations respectively. Mr. Thurmann has 
shown how remarkably this fact holds true in the Bernese 
Jura, although the argillaceous divisions, so conspicuous in 
England, are feebly represented there, and some entirely 
wanting. 
The calcareous portion of the Great Oolite consists of sev¬ 
eral shelly limestones, one of which, called the Bath Oolite, 
is much celebrated as a building-stone. In parts of Glouces¬ 
tershire, especially near Minchinhampton, the Great Oolite, 
says Mr. Lycett, ^^must have been deposited in a shallow 
sea, where strong currents prevailed, for there are frequent 
changes in the mineral character of the deposit, and some 
beds exhibit false stratiflcation. In others, heaps of broken 
shells are mingled with pebbles of rocks foreign to the neigh¬ 
borhood, and with fragments of abraded madrepores, dicot¬ 
yledonous wood, and crabs’ claws. The shelly strata, also, 
have occasionally suffered denudation, and the removed por¬ 
tions have been replaced by clay.” In such shallow-water 
