THE FOSSILIFEROUS ROCKS. 33 



manganese, which were found by Sir Wyville Thomson, in the 

 Challenger, to be so numerously scattered over the floor of the 

 Pacific at great depths. In accordance with this mode of 

 origin, it is exceedingly common to find in the center of all these 

 nodules, both old and new, some organic body, such as a bone, 

 a shell, or a tooth, which acted as the original nucleus of pre- 

 cipitation, and 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 micro- 

 scopical 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 the 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 importance, 

 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 ; which, nevertheless, are essentially 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 proportion 

 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 consider- 

 able deposits of siliceous matter by the mere accumulation of 

 their skeletons. Amongst the animals which require special 

 3 



