PROF. W. 0. WILLIAMSON ON MINKIIA I.IZA IK t\. 189 



closed cavities we have two distinct conditions, someti me.'< in 

 the same cavity. I have at home a magnificent specimen of a 

 Nautilus from the Mountain Limestone of Craven, in which all 

 the closed chambers characteristic of the posterior part of that 

 spiral shell are filled with fine crystalline forms of carbonate of 

 lime or calcareous spar. This condition is very common 

 amono:st the fossils found in the limestone rocks. 



But, on the other hand, I could show you some fossil 

 Calamites from the coal measures where the first deposits in 

 the interior of a large cavity, from which the pith has dis- 

 appeared, consist of beautiful layers of the fibrous form of 

 lime, known as Arragonite ; but after a while this formation 

 has ceased, and the solution in the centre of the cavity has 

 shot into crystals of calcareous spar. 



But cases of a less simple kind are much more numerous. 

 In these the cavity created by the disappearance of the im- 

 bedded organism becomes filled with other foreign ingredients ; 

 these are chiefly lime, silica, and iron. Often no traces of the 

 original object are preserved. A geologist applying his 

 hammer to stones of this kind would, on breaking up the 

 matrix, find a pseudomorph of the object originally enclosed 

 within it, but which instead of being composed as the original 

 was, say of lime, was now composed of flint or iton. 



Another branch of our subject closely allied to that now 

 under consideration is that of the protracted preservation and 

 ultimate transformation of the fossilized objects. It is very 

 difficult to understand how, in many cases, objects primarily 

 prone to decay, resisted that tendency sufficiently long to become 

 replaced by mineral matter. We have already seen that in 

 many instances they were not preserved long, but in others, as 

 in many of our Carboniferous fossil plants, every minute tissue 

 is preserved almost exactly as it was when living. As a rule it 

 is probable that the process of decay is impeded by the complete 

 exclusion, in many such^^submarine conditions, of all atmospheric 

 influence, whether organic or chemical. Anyhow the process 

 was often arrested sufficiently long to admit of the perfect pre- 

 servation of the most delicate tissues. 



When I was a youth another important observation was 

 made by Professor Turner, the then distinguished chemist. He 

 found that the replacement of organic substances by mineral 

 JouRN. Q. M. C. Series II., No. 32. 13 



