32 INTRODUCTION. 



degree of induration. Indeed, even in a single hand-specimen, it is 

 not unusual to find that some portions of the rock have undergone 

 complete secondary crystallisation, while others are comparatively 

 unchanged. 



Siliceous Organic Rocks. — We have seen that the calcareous or 

 lime-containing rocks are the most important group of organic de- 

 posits, while the siliceous or flint-containing rocks may be regarded 

 as the most important, most typical, and most generally distributed 

 of the mechanically formed deposits. We have, however, now briefly 

 to consider certain deposits which are more or less completely formed 

 of flint, but which nevertheless are essentially organic in their origin. 



Silica is probably invariably held in solution in small quantity in 

 natural waters, whether these be fresh or salt. Small as is the quan- 

 tity of silica dissolved by rivers or by the sea, there is sufficient of 

 it to supply material for the flinty skeletons of innumerable organ- 

 isms, both animal and vegetable ; and the accumulation of such 

 skeletons may, under favourable conditions, give rise to very con- 

 siderable deposits of siliceous matter. The two principal groups of 

 animals which secrete a siliceous skeleton, and may thus produce 

 deposits of silica, are certain forms of the Sponges and the minute 

 organisms known as the Polycystina. In a very large number of 

 Sponges, the skeleton consists of variously shaped needles or " spi- 

 cules " of flint, sometimes detached and entirely separate, at other 

 times more or less closely united with one another. The accumu- 

 lation of these spicules at the bottom of the sea may give rise to 

 extensive siliceous deposits, such as have been described by Dr 

 Hinde as occurring in the Lower and Upper Greensand of Britain. 

 In some cases the skeletal structures of Sponges which have been 

 accumulated to form deposits such as those above alluded to, have 

 undergone comparatively little change, and their presence can be 

 readily recognised. In other cases, however, these siliceous struc- 

 tures have undergone much alteration, and their existence cannot be 

 demonstrated without difficulty. It is known, namely, that there is 

 a marked difference as regards relative solubility in water between 

 ordinary crystalline quartz, on the one hand, and the peculiar form 

 of silica which occurs in the skeletons of animals and plants, on the 

 other hand. Quartz is relatively a very stable substance, and it is 

 only in highly heated waters, containing in solution such ingredients 

 as the alkaline carbonates (as, for example, in the waters of certain 

 hot springs), that quartz is ever found to be dissolved in large quan- 

 tity. On the other hand, the silica which forms the skeleton of flint- 

 secreting animals and plants exists under a peculiar modification 

 — as " amorphous " or " colloidal " silica — which is comparatively 

 unstable, and, under suitable conditions, freely soluble in water. 

 Hence, siliceous deposits formed by the accumulation of the flinty 



