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the lime salts existing in sea water, especially upon the sulphate of 
lime. This results in a small precipitate of carbonate of lime, 
which descends amongst the organic remains, and eventually forms 
the paste that binds the whole together into rock. Seeing that 
every fathom of sea water contains more or less decomposing 
organic matter of the natures referred to, a steady, if slow, precipi- 
tation of carbonate of lime must be always in progress of formation. 
Therefore, while the greater part of every limestone of marine 
origin is due directly to organic agencies, chemical action is 
accountable for the remainder. Concisely stated, the sequence of 
events resulting in the formation of limestone is as follows :— 
(1) Liberation of lime salts by the action of Hz COs: from a 
land surface. 
(2) Transportal in solution seawards. 
(3) Conversion into CaSOx, and other lime salts. 
(4) Diffusion of these through oceanic waters. 
(5) Assimilation of these lime salts by organic agencies, which 
reconvert these lime salts into CaCOs (generally with the 
crystalline form of aragonite) which is subsequently left on 
the sea bottom. 
(6) Concurrent precipitation of CaCOg by the action of dead 
organisms, &c., upon CaSOx4 
(7) Cementation of the compound. 
(8) Conversion of the unstable form aragonite into the stable 
calcite. 
(9) Upheaval, and consequent formation of joints in the rock. 
(10) Further upheaval, exposure to denuding agents, and so 
through the same cycle of changes over again. 
The following table exhibits the chief modes of origin of lime- 
stones :— 
CLASSIFICATION OF LIMESTONES ACCORDING TO THEIR ORIGIN. 
Conditions of Formatioi. 
A. Subaqueous. 
(a) Organic agencies more prominent than the chemical, 
a 
