CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 1 5 



vention of living beings ; though it is possible that in some cases the 

 chemical changes which have resulted in the production of masses 

 of gypsum may have been secondary, and may have acted at some 

 period posterior to the original deposition of the rocks associated 

 with these. 



Another lime-salt which owes to chemical action its present form, 

 and its present relations to the rocks with which it is associated, is 

 phosphate of lime. Calcium phosphate occurs in the form of larger 

 or smaller crystals (apatite) in many crystalline rocks, whether these 

 be metamorphic or igneous in origin. It also sometimes occurs in 

 considerable beds (phosphorite) in formations of various ages ; and 

 it occurs abundantly in the form of nodules in some parts of the 

 Secondary and Tertiary deposits. It likewise may occur dissem- 

 inated through the ordinary stratified rocks in such a condition as 

 not to be capable of detection save by chemical analysis, as has 

 been shown by Dr Hicks in the case of the Cambrian rocks of 

 Wales. When it is found in the crystalline or in the massive con- 

 dition, there is no reason to doubt that calcium phosphate is the 

 product of direct chemical action. Even in these cases, however, it 

 is quite possible that it may have been sometimes derived in the 

 first place from the skeletons or excrements of animals. Phosphate 

 of lime forms the larger proportion of the earthy matter of the bones 

 in Vertebrate animals, and also occurs in less amount in the skeletons 

 of certain of the Invertebrates (e.g., Lingula and Discina, among the 

 Brachiopods ; Conularia and Hyolithes, among the Pteropods ; and 

 the Crustacea in general). Phosphate of lime is thus, perhaps even 

 more distinctively than carbonate of lime, an organic compound. 

 When calcium phosphate occurs minutely disseminated through a 

 rock, it is tolerably certain that it has been derived from animals 

 and plants. It is also almost certain that the phosphate of lime in 

 the so-called " coprolites " of the Cambridge Greensand, as in other 

 similar phosphatic nodules, is organic in origin. Some of these 

 nodules consist of organisms, such as Sponges, infiltrated with phos- 

 phate of lime, but most of them would seem to have been formed by a 

 process of segregation similar to that which has given rise to nodules 

 of clay-ironstone or of carbonate of lime in beds of shale. The 

 name of " coprolites " given to these phosphatic nodules is founded 

 upon a misconception, as they are not actually the fossilised excre- 

 ments of animals. In various formations, however, there are found 

 genuine " coprolites " — i.e., the petrified excreta of Fishes, Reptiles, 

 or Mammals, — and these are largely composed of phosphate of lime. 



By far the largest and most important group of the chemically- 

 formed rocks is that of the Calcareous Rocks, comprising all those 

 rocks in which carbonate of lime is the predominating ingredient, 

 and which are therefore spoken of by the general name of Hmesto7ies. 



