18 INVERTEBRATE PALAEONTOLOGY 



moulds will be left, representing the fossils in negative 

 (see PL vi.). But if solution takes place before the 

 sand has become consolidated, the grains close in to 

 fill up the gaps, and no traces of fossils may remain. 

 The soluble mineral matter is carried away by perco- 

 lating water, to be redeposited elsewhere as a cement 

 between the sand-grains or as a filling for cracks in 

 neighbouring rock-masses. Further, the intense hardness 

 of the quartz and other grains, when compared with 

 that of calcite or aragonite, causes application of even 

 gentle pressure to destroy the finer details of shells. 

 The resulting pitted surface, often with difficulty dis- 

 engaged from the penetrating sand, bears little or no 

 resemblance to that of the original shell, even if 

 the fossil has escaped comminution in its uncongenial 

 surroundings. 



Clay, being made of minute flakes of soft, micaceous 

 material, and being practically impervious to water, has 

 neither of the disadvantages of sand as a matrix for 

 preservation of fossils. In such deposits as the Gault 

 or the Barton Clay, fossils often retain their original 

 "mother-of-pearl" sheen, and present all the details 

 of their ornament as clearly as recent shells. Even 

 chitinous or ligamentous tissues may be preserved 

 almost unchanged. But in spite of these good points, 

 argillaceous rocks are liable to develop two qualities that 

 have a baneful influence on their contained organic 

 remains. The flaky and waterlogged condition of the 

 materials that give the "clay" character can persist 

 only in uncompressed silt. Prolonged deposition brings 

 the earlier formed parts under the weight-pressure of 

 overlying sediment, with the result that the small flakes 

 are rearranged in parallel layers, and included water is 

 squeezed out. The shale thus formed shows great 

 reduction in thickness from its previous bulk, and com- 



