CALCAREOUS ACCUMULATIONS 211 



the chemical effects of dead plants, and not by the activities of 

 living ones, it will be best to retain the iron precipitates with the 

 chemically formed rocks. Important as they are from an economic 

 point of view, iron-ore beds are not extensive constituents of the 

 earth's crust, and should be regarded as minerals rather than as 

 rocks. 



Ironstone is a general name for the various ores. Haematite 

 occurs in strata and filling the cavities in limestone rocks. Limon- 

 ite also occurs in strata, and at the present day is precipitated 

 from solution in lakes. Siderite is found in beds, or mixed with 

 chert or in clay concretions. Magnetite is associated with crys- 

 talline rocks, in which it often forms great masses and beds. 



3. ORGANIC ACCUMULATIONS 



The organically formed rocks are those whose materials were 

 accumulated by living beings, on the death of which more or less 

 of their substance was preserved, added to by successive genera- 

 tions, and finally compacted into rock. In preceding chapters 

 we have read of these processes as going on at the present 

 time, in peat bogs, in the coral reefs, shell banks, limestone pla- 

 teaus, and organic oozes of the ocean. Similar processes have 

 been at work in all ages of the earth's history since the first 

 appearance of living things, and very extensive rocks have thus 

 been built into the solid crust of the globe. An exact classifica- 

 tion would require us to place certain of those rocks among the 

 mechanical sediments, because the actual work of accumulation 

 was performed by mechanical agencies, such as waves and cur- 

 rents. But it will be more convenient to examine together all 

 those rocks which are principally made up of organic materials, 

 especially as it is not always easy to distinguish the results of one 

 mode of formation from those of the other. 



a. Calcareous Accumulations 



Limestone is a very abundant, important, and widely distributed 

 rock, the commonest of the organic accumulations. It is com- 

 posed of carbonate of lime in varying degrees of purity, hardness, 



