22 INTRODUCTION. 



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 dis- 

 tributed of the mechanically-formed rocks. We have, how- 

 ever, now briefly to consider certain deposits which are more 

 or less completely formed of flint ; but 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 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 Polycystina. These little creatures are 

 of an extremely low grade of organisation, very closely 

 related to the 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 ; 

 and their shells are present in some numbers in the fora- 

 miniferal ooze which is found at great depths in the Atlantic 

 and Pacific oceans, being easily recognised by their exquisite 

 shape, their glassy transparency, the general presence of 

 longer or shorter spines, and the sieve- like perforations in 



