PRELIMINARY CONSIDERATIONS. 



IS 



spicules of sponges have been replaced by carbonate of lime. 

 The organic matter present in the fossils greatly promotes these 

 changes, by the substances produced in its decay, and thus it 

 often happens that the shells, corals, etc., contained in lime- 

 stone have been replaced by flint, while the in-losing lime- 

 stone is unchanged. Fig. lo shows the various conditions 

 which a coral myy assume under these different modes of 

 treatment. 



a 



.•I i; 



I/I 



1 

 » 



I 



» 



.9 



« 



5 



,litf » 



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I 

 t 

 t 

 I 



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ii I I It I 



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< vy/yy/.ii 



Fig. io.— Diagram showing different state of 



ssilisation of a cell of a tabulate coral 



(Dawson's Dawn of Life) 



a Natural condition, wall calcite cell empty, b Wall calclte, cells filled with the same. 

 c Walls calcite, cells filled wuh silica or a silicate, d Wall silicified cells filled 

 with calcite. e Wall silicified, cell filled w.th silica. siiicmea, cells tilled 



The substance of a fossil may be entirely removed by decay 

 or solution, leaving a mere mould representing its external 

 form, and this may subsequently be filled with mineral matter, 

 so as to produce a natural cast of the object. This is very 

 common in the case of fossil plants; and large trunks of trees 

 may sometimes be found represented, as seen in Fig. ii, by 

 stony pillars retaining nothing of the original wood except 

 perhaps a portion of the bark in the state of coal. It some- 

 times happens that the substance of fossils has been removed, 

 leaving mere empty cavities, sometimes containing stony cores* 

 representing the internal chambers of the fossils. Again, cal- 

 careous fossils imbedded in hard rocks are often removed by 

 weathering, leaving very perfect impressions of their forms. 

 For this reason the fossil remains contained in some hard 



