MATERIALS 19 



pression is bound to involve all but the most massive of 

 its " extraneous " contents. Hence fossils preserved in 

 shales are normally flattened, the small quantity of clay 

 that filled their cavities being reduced to an almost 

 negligible film (see PL iv. fig. 4). Not only does this 

 process result in loss of form, but it frequently causes 

 the two sides of the fossil to be telescoped into one 

 another, with very confusing effect. The difficulty of 

 reconstruction of extinct types whose detailed structure 

 cannot be deduced from study of living relatives is 

 greatly increased when the characters of both surfaces 

 are thus combined. Painful examples of this un- 

 fortunate type of preservation are afforded by the Sea- 

 Urchins and Starfish found in argillaceous flagstones of 

 the Lower Ludlow series. Secondly, the impermeable 

 nature of a clay matrix, while effectively excluding 

 destructive action from without, no less efficiently 

 encourages chemical changes within the rock. Even 

 the purely soft tissues of organisms may be buried in 

 the silt, and by their slow decay give rise to various 

 volatile substances, such as sulphuretted hydrogen, that 

 strongly affect minerals doomed to association with 

 them. The production of Marcasite (see PI. v. fig. 2) 

 (an unstable form of Iron sulphide) and Selenite 

 (Sulphate of lime) are two frequent results of such 

 reactions, and often these two wholly crystalline minerals 

 afford the sole evidence of previous existence of fossils 

 in clay. Nevertheless, in spite of these two serious 

 disadvantages, clay is the best medium for the preserva- 

 tion of fossils. 



A calcareous matrix is no less porous than an 

 arenaceous one, but the similarity between its physical 

 properties and those of the contained organic remains 

 (supposing these to be made of calcite) mitigates the 

 dangers to which the latter are subject. Although the 



