MATERIALS 41 



encased in concretions, and even Trilobites and Grapto- 

 lites are occasionally found unaltered in form, and 

 hardly changed in composition. Remarkable specimens 

 of Eurypterus found in the Silurian of the Isle of Oesel 

 (North Russia) are sometimes so perfectly preserved 

 that the carapaces can be detached from the marl and 

 examined by transmitted light. Lenses of compound 

 eyes of Trilobites may also retain their transparency. 



A mode of preservation in which the original tissue 

 is retained, impregnated with fresh mineral-matter, is 

 more usual than that indicated above, and perhaps more 

 satisfactory on the whole. The majority of shells, and 

 especially the skeletons of Echinoderms, have a fairly 

 open texture, and precipitation of new mineral deposits 

 in the pores strengthens the fabric to a degree surpass- 

 ing that attained during life. Mineral impregnation is 

 most frequent in calcite fossils embedded in calcareous 

 matrices, and is shown most clearly in Echinoderm 

 tests. The crystalline continuity of the fresh material 

 with that of the original tissue may lead to dis- 

 advantages by obscuring structural outlines or inducing 

 cleavage. The former difficulty is, however, usually 

 mitigated by slight coloration of the secondary in- 

 filling, while the latter may be helpful in the study 

 of tests where each ossicle is independently orientated. 

 Occasionally the impregnating mineral may be different 

 from that composing the test, as in fossils that are 

 embedded in, but not replaced by, flint. Such specimens 

 reveal minute structure with admirable definition when 

 viewed in thin sections. 



The process of infilling, which is strictly comparable 

 with the cementation of arenaceous or calcareous rocks, 

 may at times be carried to excess, so that incrustations 

 of new crystalline mineral cover the surface of the fossil. 

 This condition is particularly common in the interior of 



