CHARACTERISTIC FOSSILS. 17 
have been destroyed by tlie influence of carbonated waters, whose 
action may be greater when the percolation is slow. Belemnites, 
Pecten?, and Oysters frequently remain when other calcareous 
organisms have perished ; but the shells of these fossils are formed 
of calcite, and, as pointed out by Dr. Sorby,* they are more stable 
than shells formed of aragonite. Limestones formed of broken 
or comminuted shells, such as the Ham Hill Stone (Inferior 
Oolite) and the Forest Marble, consist largely of fragments of 
Pecten and Ostrea. 
In many brickyards the fossils are most abundant in the deeper 
part of the working, and sometimes they appear' only in such 
situations. This is irrespective of any special horizons, and is no 
doubt due to the fact that specimens from the higher exposed 
strata have been obliterated by meteoric agencies. This loss is 
sometimes occasioned by the decomposition of iron-pyrites and 
the formation of selenite from the calcareous matter of the fossils. t 
Belemnites, however, are more frequently preserved near the 
surface than some other forms. In pyritic clays the shells of 
Ammonites and other fossils are sometimes entirely replaced by 
pyrites. 
Many of the fossils found in clays are much compressed, and 
uncrushed forms must be sought in the septaria and other nodules 
that may occur in the formation. The varying thickness of clay- 
beds in some localities may be due to unequal compression of 
the strata. 
Characteristic Fossils Fossil-beds Zones. 
The fossil contents of each formation vary as might be expected 
in different localities, even on the same approximate horizon. 
Fossils are more or less sporadic in their mode of occurrence. 
Beds are comparatively barren^ or rich in fossils within short 
distances. In one area Cephalopoda prevail, in another Lamelli- 
bnmchs, in a third Brachiopoda, in a fourth Corals, and so on. On 
this account it is impossible to give lists of the characteristic and 
abundant fossils of any formation that are of more than general 
vsilue ; some species being common in one or more areas and rare 
in others. This is the case with the species of Cardinia and 
Hippcpvdium in the Lias, with the Ammonites and Brachiopods 
of the Inferior Oolite, &c., and it is natural, especially with gre- 
garious forms or those dependent on certain conditions of sea-bed, 
&c. Thus we find many fossil-beds at various horizons ; some 
characterised by one species, others by two or more species of the 
same genus, or by several genera. 
It appears best to designate these beds by the generic as well as 
the specific name of the prominent fossil, although this plan has 
not always been adopted. In the Inferior Oolite certain species 
are prevalent at various horizons, such as Clypeus Plotii, Tere- 
* Address to Geol. Soc., 1879 ; V. Cornish and P. F. Kendall, Geol. Mag., 1888, 
p. 6G. 
t See Duncan, Quart. Journ. Geol. Soc., vol. sxii. p. 12. 
E 70859, B 
