CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 1 7 



dissolved lime than fresh waters, in spite of the circumstance that 

 rivers are constantly pouring into the sea vast quantities of this 

 substance. 



Considering the constant production of carbonate of lime by vari- 

 ous animals and plants, it is not surprising to find on investigation 

 that many limestones are more or less extensively composed of the 

 skeletons of living beings. Most limestones are therefore, more or 

 less clearly, organic rocks. There are, however, two methods — not 

 always very clearly separated from one another — in which an organic 

 limestone may be formed. In one set of cases, the limestone is the 

 result of the accumulation of the calcareous skeletons of animals in 

 the place where these organisms actually lived and grew. This is 

 seen occasionally where a limestone has been formed by the growth 

 of innumerable generations of sedentary Molluscs, such, for example, 

 as Oysters. Some Crinoidal limestones have also been formed by 

 the accumulation of Crinoids in place ; and many of the more 

 modern coral-limestones are similarly the result of the growth of 

 the lime-producing polypes in the locality where we now find the 

 rock. In another and more extensive set of cases, the limestone has 

 been formed by the gradual accumulation of the skeletons of animals 

 or plants which lived in some place more or less widely removed 

 from that occupied by the limestone itself. Thus, extensive calcare- 

 ous deposits may be formed at the bottom of the deep sea by the 

 slow accumulation of the calcareous skeletons of animals which live 

 at the surface of the ocean, and the shells of which fall to the bottom 

 on the death of the animal which produced them. This is seen 

 in certain Foraminiferal limestones and in Pteropodal limestones, 

 though in all such cases the rock is in part made up of the skeletons 

 of animals which actually lived at the bottom of the sea. In other 

 cases, the calcareous skeletons of animals are thrown up in great 

 banks by the action of the sea in the neighbourhood of land. This 

 is the case, for example, with the great accumulations of shell-sand 

 on many parts of our shores, or with the still more extensive deposits 

 of coral-sand in warm seas. In such cases, the limestone is so far 

 organic that it is formed mainly out of the skeletons of lime-secreting 

 animals or plants, but it is also so far mechanical that the actual 

 formation of the limestone has been due to the breaking up and 

 wearing down of these skeletons by the movements of the waves 

 of the sea. 



If this wearing-down action has been sufficiently long-continued 

 and sufficiently complete, we may not be able to recognise in the 

 limestone many, or indeed any, actual fragments of shells or other 

 animal structures ; but the rock may appear under the microscope as 

 a fine-grained calcareous mud, made up of minute, mostly non-crys- 

 talline granules of carbonate of lime (fig. 2, a). This is the case, for 



vol. 1. B 



