J. G. Goodchild — The Paste of Limestones. 75 



over the ocean-floor, where it subsides as a thin stratum of impalpable 

 calcareous mud. Since the time that coral animals assumed the 

 reef-building habit (which many think was not earlier than Mid- 

 Tertiary times) such a factor must bave been an important one in 

 the production of the paste in question. Even in Pre-Tertiary 

 times, before tbere were any coral reefs, the wear and tear of other 

 calcareous organisms must have contributed to the same result. But 

 as such action, from the nature of the case, must have been confined 

 to littoral regions, it can hardly be taken much into account in 

 dealing with the present question. It is quite clear in the case of 

 the older limestone that the paste did not result from the detrition, 

 on the spot, of the calcareous exuviae existing on the sea-bottom ; 

 for, be the calcareous organisms what they may, they retain their 

 original sculpturing, or other ornamentation, in as perfect a con- 

 dition, so far as wear is concerned, as it was during the lifetime of 

 the animals. In the course of several years' close study of every- 

 thing relating to limestones, in the field and in the study, I have 

 never yet come across a single instance where any evidence whatever 

 of such attrition could be made out. 



Another explanation sometimes given is that, when calcareous 

 organisms have lain for any length of time on the sea-floor, they are 

 attacked by chemical agencies, and are thereby dissolved more or 

 less — the solution being, under different conditions, redeposited as 

 a chemical precipitate. Without questioning the validity of this 

 theory in particular cases, it will suffice to remind the reader that 

 many of the facts tell strongly against such a view. When animals 

 build up a calcareous framework, the carbonate of lime is invariably 

 mixed with more or less organic matter, allied to chitin in many 

 of its properties. The function of this substance, as Bischoff long 

 since pointed out, is to enable the hard parts of the animals to resist 

 the attacks of agents tending to bring about any such corrosion. 

 As a matter of fact it is usually not until long after the animal has 

 been dead, and its shell has been exposed to the attacks of subaerial 

 erosion, that it begins to go to pieces at all. As a consequence, it 

 is as rare to find a corroded shell, for example, in any limestone 

 clearly of marine origin, as it is to find amongst recent shells the 

 apex of an aged Gasteropod, or the umbo of a full-grown Lamelli- 

 branch, showing any sign of corrosion. It is only where the forces 

 connected with subaerial denudation come into play that any such 

 corrosion appears to be possible. 



Another factor that may contribute to form some of the paste of 

 limestone is the agency of certain fishes and molluscs that feed 

 upon organisms secreting calcareous frameworks. Part, at least, of 

 the finely-comminuted stony matter swallowed by these predatory 

 animals is subsequently voided, and eventually subsides to the 

 bottom. From the very nature of the case this can hardly ever 

 have been of much importance in the present connection, although 

 it cannot well be left altogether out of account. 



Other explanations also have been from time to time advanced ; 

 but not one of them has proved altogether satisfactory. 



