32 PRINCIPLES OF PALAEONTOLOGY. 



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 

 m'croscopical 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 palaeontological 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 siliceous 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; 



