MATERIALS 45 



touched. In favourable cases details of form, both 

 internal and external, can be exposed as clearly as if 

 the fossil had never been encased by rock. Exquisite 

 preparations of Brachiopod brachidia (PI. xi. fig. 9) 

 and Blastoid hydrospires can be made in this way ; 

 but alas ! siliceous petrifaction of Invertebrates is rarely 

 found in this country. 



Calcite often replaces Aragonite in limestones, and 

 may even form pseudomorphs after opaline sponge- 

 spicules. Iron Pyrites, as Marcasite or Pyrite, is the 

 usual petrifying mineral for argillaceous rocks. It is 

 difficult in this case to determine whether the replace- 

 ment has been subsequent to removal of the original, 

 or by molecular replacement, since the opacity of the 

 mineral hides any minute structure that may be pre- 

 served. Marcasite has serious disadvantages as a 

 petrifying substance, since it is extremely unstable 

 when exposed, and has a prismatic habit that may 

 reveal itself in nodular outgrowths independent of the 

 original cavity (PL v. fig. 2). Many Ammonites and 

 other (usually Aragonite) shells from the Lias, Oxfordian 

 and Gault are represented by Marcasite pseudomorphs 

 that are obscure in themselves, and difficult to preserve 

 after extraction. Pyrite, which is more characteristic 

 of older argillaceous rocks, is quite stable, but its pro- 

 nounced cubic crystallization may cause destruction 

 of surface details. However, in some instances, it 

 proves an admirable replacing mineral, since its hard- 

 ness is usually greater than that of the matrix. The 

 Devonian Starfish from Bundenbach, which can be 

 cleared of their slaty covering with a brass-wire brush, 

 show Pyrite in its most satisfactory r61e as a petrifying 

 mineral. 



In Sandstones, where the substance of the fossils is 

 usually the only soluble material, petrifactions may be 



