OF FOSSILS. 286 



Siliceous nuclei frequently fill entirely the in- 

 ternal cavity of organic fofiils, and fomctimes 

 even the fame matter furrounds their external 

 furfact*. I am in poflelTion of an cchnitcs, the 

 Jhell of which is filled witli cainmon flint, and 

 Ihcws upon the lurface of the nucleus all its na- 

 tural inequalities ;' the (hell itfeif, however is 

 calcareous and fpataceous, although it was iin- 

 heded in (iliceous earth on both fules. Small 

 Ihells occur fomctimes in jafper, but very rarely *, 

 and are not more frequent in petrolilex. 



Organic bodies, themfelves aifo are found 

 penetrated with iiliccous matter. Siliceous pe- 

 trcfadions of the trunks of trees are often clif- 

 tindly marked with the growth of every year. 

 Siliceous mulcles and cockles alfo frequently 

 occur, and final 1 corals even arc fometimcs clear- 

 ly to be dillinguiflicd in common flints. 



I have fee n the marks of leaves accurately 

 exprelled in quartz, and the cpitomium of Blan- 

 kcnburg is often quartofe. 



Nuclei of land are fometimcs to be met with; 

 but the figure of their furface is generally fo ob- 

 I cure, that it is very difficult to determine from 

 what organic body they were produced. 



In the fand pit at iMaeftricht was found not 

 long ago the (keleton of a crocodile, fome teeth 

 of which were lent to me. 



T i CLXX. 



* 1'VrWr iu Epiil. <U- Italia. 



