32 PRINCIPLES OF PALZONTOLOGY. 
was thus preserved in a shroud of mineral matter. Many 
nodules, it is true, show no such nucleus; but it has been 
affirmed that all of them can be shown, by appropriate 
microscopical investigation, to have been formed round an 
original organic body to begin with (Hawkins Johnson). 
The last lime-salt which need be mentioned is gypsum, or 
sulphate of lime. This substance, apart from other modes of 
occurrence, is not uncommonly found interstratified with the 
ordinary sedimentary rocks, in the form of more or less irregu- 
lar beds; and in these cases it has a paleontological import- 
ance, as occasionally yielding well-preserved fossils. Whilst 
its exact mode of origin is uncertain, it cannot be regarded as 
in itself an organic rock, though clearly the product of chemical 
action. ‘To look at, it is usually a whitish or yellowish-white 
rock, as coarsely crystalline as loaf-sugar, or more so; and the 
microscope shows it to be composed entirely of crystals of 
sulphate of lime. 
We have seen that the calcareous or lime-containing rocks 
are the most important of the group of organic deposits; whilst 
the sz/iceous or flint-containing rocks may be regarded as the 
most important, most typical, and most generally distributed 
of the mechanically-formed rocks. We have, however, now 
briefly to consider certain deposits which are more or less 
completely formed of flint ; but which, nevertheless, are essen- 
tially organic in their origin. 
Flint or silex, hard and intractable as it is, is nevertheless 
capable of solution in water to a certain extent, and even of 
assuming, under certain circumstances, a gelatinous or viscous 
condition. Hence, some hot-springs are impregnated with 
silica to a considerable extent ; it is present in small quantity 
in sea-water; and there is reason to believe that a minute pro- 
portion must very generally be present in all bodies of fresh 
water as well. It is from this silica dissolved in the water that 
many animals and some plants are enabled to construct for 
themselves flinty skeletons; and we find that these animals and 
plants are and have been sufficiently numerous to give rise to 
very considerable deposits of siliceous matter by the mere 
accumulation of their skeletons. Amongst the animals which 
require special mention in this connection are the microscopic 
organisms which are known to the naturalist as Polycystina. 
These little creatures are of the lowest possible grade of organ- 
isation, very closely related to the animals which we have pre- 
viously spoken of as Foraminifera, but differing in the fact that 
they secrete a shell or skeleton composed of flint instead of 
lime. The Polycystina occur abundantly in our present seas ; 
