46 INVERTEBRATE PALAEONTOLOGY 



made of a great variety of minerals. Silica (often as 

 Quartz) and Calcite still predominate, but metallic 

 minerals may often form the replicas. Malachite, 

 Blende, Hematite, Limonite and Galena may be cited 

 as minerals occasionally found replacing fossils in sand- 

 stones (and even in limestones) ; while Barytes, Celestine 

 and Selenite have been found as organic pseudomorphs 

 in greywackes and clays. 



(b) By molecular replacement 



It is clear that the type of petrifaction above de- 

 scribed can preserve no vestiges of the minute structure 

 of the fossils replaced. Save for the peculiar character 

 of the shape of the cavity available, the secondary 

 mineral follows its own crystalline mode of accretion 

 as faithfully in an organic cast as in a fissure or geode. 

 But in some cases deposition of the replacing mineral 

 takes place part passu with removal of the original. 

 Under such circumstances replacement is carried out 

 with great perfection, so that the finished pseudomorph 

 possesses every detail of microscopic structure of the 

 fossil. Such accurate replacements are common among 

 minerals (e.g. Quartz after Crocidolite) and in the 

 silicification of woody tissue ; and they are not unknown 

 among Invertebrate fossils. Shells and tests from the 

 Upper Chalk may sometimes show patchy petrifaction 

 by silica, especially when associated with a flint-nodule, 

 but the replacement is rarely complete. 



(IV) DISTORTION AND DISFIGUREMENT (PL iii. fig. 3) 



Perfect specimens of fossils, even of the commonest 

 kinds, are relatively rare. To the numerous abnormali- 

 ties, diseases and accidents that may mar an organism 



